Our business is based on a recurring revenue model comprised of colocation and related interconnection and managed infrastructure offerings. We consider these offerings recurring because our customers are generally billed on a fixed and recurring basis each month for the duration of their contract, which is generally one to three years in length. Our recurring revenues have comprised more than 90% of our total revenues during the past three years. In addition, during any given quarter of the past three years, more than half of our monthly recurring revenue bookings came from existing customers, contributing to our revenue growth. During the three months ended September 30, 2018 and 2017, our largest customer accounted for approximately 3% and 4%, respectively, of our recurring revenues. During the nine months ended September 30, 2018 and 2017, our largest customer accounted for approximately 3% of our recurring revenues. Our 50 largest customers accounted for approximately 38% and 37%, respectively, of our recurring revenues for the three months ended September 30, 2018 and 2017. Our 50 largest customers accounted for approximately 38% and 36%, respectively, of our recurring revenues for the nine months ended September 30, 2018 and 2017.
Our non-recurring revenues are primarily comprised of installation services related to a customer’s initial deployment and professional services we perform. These services are considered to be non-recurring because they are billed typically once, upon completion of the installation or the professional services work performed. The majority of these non-recurring revenues are typically billed on the first invoice distributed to the customer in connection with their initial installation. However, revenues from installation services are deferred and recognized ratably over the period of contract term. Additionally, revenue from contract settlements, when a customer wishes to terminate their contract early, is generally treated as a contract modification and recognized ratably over the remaining term of the contract, if any. As a percentage of total revenues, we expect non-recurring revenues to represent less than 10% of total revenues for the foreseeable future.
The largest components of our cost of revenues are depreciation, rental payments related to our leased IBX data centers, utility costs, including electricity, bandwidth access, IBX data center employees’ salaries and benefits, including stock-based compensation, repairs and maintenance, supplies and equipment and security services. A majority of our cost of revenues is fixed in nature and should not vary significantly from period to period, unless we expand our existing IBX data centers or open or acquire new IBX data centers. However, there are certain costs that are considered more variable in nature, including utilities and supplies that are directly related to growth in our existing and new customer base. We expect the cost of our utilities, specifically electricity, will generally increase in the future on a per-unit or fixed basis, in addition to the variable increase related to the growth in consumption by our customers. In addition, the cost of electricity is generally higher in the summer months, as compared to other times of the year. To the extent we incur increased utility costs, such increased costs could materially impact our financial condition, results of operations and cash flows. Furthermore, to the extent we incur increased electricity costs as a result of either climate change policies or the physical effects of climate change, such increased costs could materially impact our financial condition, results of operations and cash flows.
Sales and marketing expenses consist primarily of compensation and related costs for sales and marketing personnel, including stock-based compensation, amortization of contract costs, marketing programs, public relations, promotional materials and travel, as well as bad debt expense and amortization of customer relationship intangible assets.
General and administrative expenses consist primarily of salaries and related expenses, including stock-based compensation, accounting, legal and other professional service fees, and other general corporate expenses, such as our corporate regional headquarters office leases and some depreciation expense.
We expect our cost of revenues, sales and marketing expenses and general and administrative expenses to grow in absolute dollars in connection with our business growth. We may periodically see a higher cost of revenues as a percentage of revenue when a large expansion project opens or is acquired, before it starts generating any meaningful revenue. Furthermore, in relation to cost of revenues, we note that the Americas region has a lower cost of revenues as a percentage of revenue than either EMEA or Asia-Pacific. This is due to both the increased scale and maturity of the Americas region, compared to either the EMEA or Asia-Pacific region, as well as a higher cost structure outside of the Americas, particularly in EMEA. While we expect all three regions to continue to see lower cost of revenues as a percentage of revenues in future periods, we expect the trend that sees the Americas having the lowest cost of revenues as a percentage of revenues to continue. As a result, to the extent that revenue growth outside the Americas grows in greater proportion than revenue growth in the Americas, our overall cost of revenues as a percentage of revenues may increase in future periods. Sales and marketing expenses may periodically increase as a percentage of revenues as we continue to scale our operations by investing in sales and marketing initiatives to further increase our revenue, including the hiring of additional headcount and new product innovations. General and administrative expenses may also periodically increase as a percentage of revenues as we continue to grow our business.
Taxation as a REIT
We elected to be taxed as a real estate investment trust for federal income tax purposes ("REIT") beginning with our 2015 taxable year. As of September 30, 2018, our REIT structure included all of our data center operations in the U.S., Canada, Japan