CoreSite Realty 10-K 9 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Feb. 07, 2018	Jun. 30, 2017
Document and Entity Information
Entity Registrant Name	CoreSite Realty Corp
Entity Central Index Key	1,490,892
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Public Float			$ 2,571.3
Entity Common Stock, Shares Outstanding		34,243,139
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments in real estate:
Land	$ 97,258	$ 100,258
Buildings and improvements	1,561,056	1,472,580
Total investments in real estate	1,658,314	1,572,838
Less: Accumulated depreciation and amortization	(473,141)	(369,303)
Net investment in operating properties	1,185,173	1,203,535
Construction in progress	162,903	70,738
Net investments in real estate	1,348,076	1,274,273
Cash and cash equivalents	5,247	4,429
Accounts and other receivables, net of allowance for doubtful accounts of $1,094 and $209 as of December 31, 2017, and December 31, 2016, respectively	28,875	25,125
Lease intangibles, net of accumulated amortization of $8,585 and $12,385 as of December 31, 2017, and December 31, 2016, respectively	6,314	9,913
Goodwill	40,646	41,191
Other assets, net	103,501	96,372
Total assets	1,532,659	1,451,303
Liabilities:
Debt, net of unamortized deferred financing costs of $4,930 and $3,550 as of December 31, 2017, and December 31, 2016, respectively	939,570	690,450
Accounts payable and accrued expenses	77,170	72,519
Accrued dividends and distributions	48,976	41,849
Deferred rent payable	9,928	7,694
Acquired below-market lease contracts, net of accumulated amortization of $5,608 and $5,439 as of December 31, 2017, and December 31, 2016, respectively	3,504	4,292
Unearned revenue, prepaid rent and other liabilities	34,867	37,413
Total liabilities	1,114,015	854,217
Stockholders' equity:
Series A Cumulative Preferred Stock 7.25%, $115,000 liquidation preference ($25.00 per share, $0.01 par value), 20,000,000 shares authorized and zero and 4,600,000 shares issued and outstanding as of December 31, 2017, and December 31, 2016, respectively		115,000
Common Stock, par value $0.01, 100,000,000 shares authorized and 34,240,815 and 33,896,771 shares issued and outstanding at December 31, 2017, and December 31, 2016, respectively	338	334
Additional paid-in capital	457,495	438,531
Accumulated other comprehensive income (loss)	753	(101)
Distributions in excess of net income	(177,566)	(118,038)
Total stockholders' equity	281,020	435,726
Noncontrolling interests	137,624	161,360
Total equity	418,644	597,086
Total liabilities and equity	$ 1,532,659	$ 1,451,303

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
CONDENSED CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts (in dollars)	$ 1,094,000	$ 209,000
Accumulated amortization, lease intangibles (in dollars)	8,585,000	12,385,000
Unamortized Debt Issuance Expense	4,930,000	3,550,000
Accumulated amortization, acquired below-market lease contracts (in dollars)	5,608,000	5,439,000
Accumulated amortization of below-market lease	$ 5,608,000	$ 5,439,000
Series A Cumulative Preferred Stock, dividend rate (as a percent)	7.25%	7.25%
Series A Cumulative Preferred Stock, liquidation preference	$ 115,000	$ 115,000
Series A Cumulative Preferred Stock, liquidation preference (in dollars per share)	$ 25	$ 25
Series A Cumulative Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Series A Cumulative Preferred Stock, shares authorized	20,000,000	20,000,000
Series A Cumulative Preferred Stock, shares issued	0	4,600,000
Series A Cumulative Preferred Stock, shares outstanding	0	4,600,000
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common Stock, shares issued	34,240,815	33,896,771
Common Stock, shares outstanding	34,240,815	33,896,771

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Data center revenue:
Rental revenue	$ 264,134	$ 218,060	$ 183,300
Power revenue	134,909	111,541	89,495
Interconnection revenue	62,293	53,077	44,234
Tenant reimbursement and other	8,637	9,086	8,295
Office, light-industrial and other revenue	11,848	8,588	7,968
Total operating revenues	481,821	400,352	333,292
Operating expenses:
Property operating and maintenance	132,820	107,212	89,805
Real estate taxes and insurance	14,913	14,250	12,144
Depreciation and amortization	129,251	108,652	95,702
Sales and marketing	18,176	17,495	15,930
General and administrative	37,548	35,369	34,179
Rent	24,125	22,631	21,075
Impairment of internal-use software			322
Transaction Costs	176	126	51
Total operating expenses	357,009	305,735	269,208
Operating income	124,812	94,617	64,084
Gain (loss) on real estate disposal			36
Interest expense	(24,147)	(12,577)	(7,098)
Income before income taxes	100,665	82,040	57,022
Income tax expense	(174)	(119)	(163)
Net income	100,491	81,921	56,859
Net income attributable to noncontrolling interests	25,636	23,212	22,153
Net income attributable to CoreSite Realty Corporation	74,855	58,709	34,706
Preferred stock dividends	(7,924)	(8,338)	(8,338)
Original issuance costs associated with redeemed preferred stock	(4,326)
Net income attributable to common shares	$ 62,605	$ 50,371	$ 26,368
Net income per share attributable to common shares:
Basic (in dollars per share)	$ 1.85	$ 1.56	$ 1.05
Diluted (in dollars per share)	$ 1.84	$ 1.54	$ 1.03
Weighted average common shares outstanding
Basic (in shares)	33,792,759	32,289,414	25,218,500
Diluted (in shares)	34,058,949	32,732,059	25,706,568

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 100,491	$ 81,921	$ 56,859
Other comprehensive income (loss):
Unrealized gain (loss) on derivative contracts	580	(1,112)	(2,426)
Reclassification of other comprehensive income to interest expense	623	1,740	1,930
Comprehensive income	101,694	82,549	56,363
Comprehensive income attributable to noncontrolling interests	25,985	23,242	21,724
Comprehensive income attributable to CoreSite Realty Corporation	$ 75,709	$ 59,307	$ 34,639

CONSOLIDATED STATEMENTS OF EQUI

CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Distributions in Excess of Net Income	Total Stockholders' Equity	Noncontrolling Interests	Total
Balance at Dec. 31, 2014	$ 115,000	$ 212	$ 275,038	$ (125)	$ (67,538)	$ 322,587	$ 335,754	$ 658,341
Balance (in shares) at Dec. 31, 2014		21,757,366
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests		$ 85	110,396	(301)		110,180	(110,180)
Redemption of noncontrolling interests (in shares)		8,500,000
Issuance of stock awards, net of forfeitures (in shares)		207,937
Exercise of stock options, net of settlements		$ 3	(2,866)			(2,863)		(2,863)
Exercise of stock options, net of settlements (in shares)		185,400
Share-based compensation		$ 1	7,632			7,633		7,633
Dividends declared on preferred stock					(8,338)	(8,338)		(8,338)
Dividends and distributions					(47,721)	(47,721)	(37,110)	(84,831)
Net income					34,706	34,706	22,153	56,859
Other comprehensive income (loss)				(67)		(67)	(429)	(496)
Balance at Dec. 31, 2015	115,000	$ 301	390,200	(493)	(88,891)	416,117	210,188	626,305
Balance (in shares) at Dec. 31, 2015		30,650,703
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests		$ 30	37,550	(206)		37,374	(37,374)
Redemption of noncontrolling interests (in shares)		3,007,000
Issuance of stock awards, net of forfeitures (in shares)		164,408
Exercise of stock options, net of settlements		$ 1	1,289			1,290		1,290
Exercise of stock options, net of settlements (in shares)		74,660
Share-based compensation		$ 2	9,492			9,494		9,494
Dividends declared on preferred stock					(8,338)	(8,338)		(8,338)
Dividends and distributions					(79,518)	(79,518)	(34,696)	(114,214)
Net income					58,709	58,709	23,212	81,921
Other comprehensive income (loss)				598		598	30	628
Balance at Dec. 31, 2016	115,000	$ 334	438,531	(101)	(118,038)	435,726	161,360	597,086
Balance (in shares) at Dec. 31, 2016		33,896,771
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests			167			167	(167)
Redemption of noncontrolling interests (in shares)		15,011
Redemption of preferred stock	$ (115,000)		4,326		(4,326)	(115,000)		(115,000)
Issuance of stock awards, net of forfeitures (in shares)		131,272
Exercise of stock options, net of settlements		$ 2	4,818			4,820		4,820
Exercise of stock options, net of settlements (in shares)		197,761
Share-based compensation		$ 2	9,653			9,655		9,655
Dividends declared on preferred stock					(7,924)	(7,924)		(7,924)
Dividends and distributions					(122,133)	(122,133)	(49,554)	(171,687)
Net income					74,855	74,855	25,636	100,491
Other comprehensive income (loss)				854		854	349	1,203
Balance at Dec. 31, 2017		$ 338	$ 457,495	$ 753	$ (177,566)	$ 281,020	$ 137,624	$ 418,644
Balance (in shares) at Dec. 31, 2017		34,240,815

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 100,491	$ 81,921	$ 56,859
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	129,251	108,652	95,702
Amortization of above/below market leases	(598)	(541)	(520)
Amortization of deferred financing costs	1,676	1,333	1,246
Gain on real estate disposal			(36)
Share-based compensation	8,946	8,892	7,114
Bad debt expense	1,338	398	201
Changes in operating assets and liabilities:
Accounts receivable	(5,087)	(13,199)	(2,146)
Deferred rent receivable	(3,645)	(7,083)	(7,799)
Deferred leasing costs	(13,315)	(14,395)	(17,555)
Other assets	(10,924)	(8,819)	(5,535)
Accounts payable and accrued expenses	1,224	4,910	6,532
Unearned revenue, prepaid rent and other liabilities	(2,235)	8,696	9,561
Deferred rent payable	2,234	(240)	(1,051)
Net cash provided by operating activities	209,356	170,525	142,573
CASH FLOWS FROM INVESTING ACTIVITIES
Tenant improvements	(7,850)	(5,878)	(7,736)
Real estate improvements	(153,956)	(293,359)	(122,196)
Acquisition of SV8	(12,158)
Acquisition of Reston Campus Expansion		(64,965)
Proceeds from real estate disposal			2,399
Net cash used in investing activities	(173,964)	(364,202)	(127,533)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercise of stock options	4,820	1,290	1,393
Proceeds from revolving credit facility	274,000	299,500	87,000
Payments on revolving credit facility	(298,500)	(247,750)	(163,250)
Proceeds from unsecured term loan	275,000	250,000	150,000
Payments of loan fees and costs	(2,410)	(2,981)	(2,315)
Payments to net settle equity awards			(4,256)
Redemption of preferred stock	(115,000)
Dividends and distributions	(172,484)	(108,807)	(87,420)
Net cash provided by financing activities	(34,574)	191,252	(18,848)
Net change in cash and cash equivalents	818	(2,425)	(3,808)
Cash and cash equivalents, beginning of period	4,429	6,854	10,662
Cash and cash equivalents, end of period	5,247	4,429	6,854
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest, net of capitalized amounts	22,252	11,528	6,219
NON-CASH INVESTING AND FINANCING ACTIVITY
Construction costs payable capitalized to real estate	32,228	28,805	37,033
Accrual of dividends and distributions	$ 48,976	$ 41,849	$ 28,104

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2017
Organization and Description of Business
Organization and Description of Business	1. Organization and Description of Business CoreSite Realty Corporation (the “Company,” “we,” “us,” or “our”) was organized in the state of Maryland on February 17, 2010 and is a fully integrated, self-administered, and self-managed real estate investment trust (“REIT”). Through our controlling interest in CoreSite, L.P. (our “Operating Partnership”), we are engaged in the business of owning, acquiring, constructing and operating data centers. As of December 31, 2017, the Company owns a 71.0% common interest in our Operating Partnership, and affiliates of The Carlyle Group and others own a 29.0% interest in our Operating Partnership. See additional discussion in Note 11, Noncontrolling Interests – Operating Partnership. Information with respect to square footage and acres of land is unaudited.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation and Basis of Presentation The accompanying consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation. Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued guidance codified in Accounting Standards Codification (“ASC”) Topic 606, Revenue Recognition — Revenue from Contracts with Customers , which amends the guidance in former ASC Topic 605, Revenue Recognition . The standard establishes a five-step model framework which recognizes revenue as an entity transfers control of goods or services to the customer and requires enhanced disclosures. This standard does not apply to leases, which will be accounted for under ASC Topic 842, Leases . The revenue standard is effective for interim and annual reporting periods beginning after December 15, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. As ASC Topic 606 does not impact lessor accounting, this standard will not significantly impact our accounting for rental revenue. In addition, there is not a significant impact to our accounting for nonlease components, including power, interconnection, and tenant reimbursement and other revenue. We adopted ASC Topic 606 on January 1, 2018, and will use the cumulative effect transition method. In February 2016, the FASB issued guidance codified in ASC Topic 842, Leases , which amends the guidance in former ASC Topic 840, Leases . We are party to leases as both a lessor and lessee. The main principle of ASC 842 requires lessees to recognize the assets and liabilities that arise from nearly all leases on the consolidated balance sheet. Lessor accounting remains mainly consistent with current guidance, with the majority of changes allowing for better alignment with the new lessee model and ASC Topic 606. The standard is effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. The standard requires a modified retrospective transition approach. We adopted ASC Topic 842 effective January 1, 2018, which would result in a transition date of January 1, 2016. The FASB has issued a Proposed Accounting Standards Update, Leases – Targeted Improvements, which proposes updates to the lease standard and would allow us to apply the new lease standard as of our January 1, 2018, adoption date and remove the requirement for us to restate our prior period financial statements upon adoption. If the proposed accounting standards update is issued in final form, we will apply the transition relief under the new lease standard as of January 1, 2018. We are currently evaluating other impacts of the FASB’s proposed updates to the lease standard, specifically the proposed practical expedient which allows lessors not to separate nonlease components from the related lease components if both the timing and pattern of revenue recognition are the same for the nonlease component(s) and related lease component and the combined single lease component would be classified as an operating lease. As a lessee the classification of our leases will not change, but we will be required to recognize a lease liability and corresponding right-of-use asset on our consolidated balance sheets for all of our operating leases. We have elected the package of practical expedients which allows us to not reassess (1) whether existing contracts contain leases, (2) the lease classification for existing leases, and (3) whether existing initial direct costs meet the new definition. We did not elect the hindsight practical expedient which permits entities to use hindsight in determining the lease term. Using the fixed noncancellable term of all existing data center leases, excluding renewal options, our initial lease liability and right-of-use asset will each be approximately $100 million on our consolidated balance sheet as of January 1, 2018. In accordance with ASC Topic 842, lessor accounting for our leases remains largely unchanged, apart from the narrower definition of initial direct costs that can be capitalized. The new standard defines initial direct costs as only the incremental costs of signing a lease. Internal sales employees’ compensation, payroll-related fringe benefits and certain external legal fees related to the execution of successful lease agreements will not meet the definition of initial direct costs under the new standard and will be accounted for as a sales and marketing expense or general and administrative expense in the consolidated statements of operations upon adoption of ASC Topic 842. For the year ended December 31, 2017, we had $1.2 million and $0.1 million of capitalized salaries and benefits and external legal fees, respectively, which would not have been capitalized if ASC Topic 842 had been in effect. On a prospective basis, we estimate that the sales and marketing expense in our consolidated statement of operations will increase by approximately $1.4 million and general and administrative expense will increase by approximately $0.1 million for the year ending December 31, 2018, as a result of the narrower definition of initial direct costs. In August 2016, the FASB issued guidance codified in ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The standard provides guidance on eight specific cash flow classification issues including debt prepayment or debt extinguishment costs, contingent consideration payments made after a business combination, and separately identifiable cash flows and application of the predominance principle. The standard will be effective for the fiscal year beginning January 1, 2018, and subsequent interim periods. We do not expect the provisions of ASU 2016-15 to have a material impact on our consolidated financial statements. In January 2017, the FASB issued guidance codified in ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. ASU 2017-04 simplifies the accounting for goodwill impairment by eliminating the process of measuring the implied value of goodwill, known as step two, from the goodwill impairment test. Instead, if the carrying amount of a reporting unit exceeds its fair value, an impairment loss shall be recognized in an amount equal to that excess, limited to the total amount of goodwill allocated to that reporting unit. The standard will be effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. We do not expect the provisions of ASU 2017-04 to have a material impact on our consolidated financial statements. In August 2017, the FASB issued guidance codified in ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. ASU 2017-12 simplifies hedge accounting by eliminating the requirement to separately measure and report hedge ineffectiveness and presenting all items that affect earnings in the same income statement line item as the hedged item. The standard will be effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. We do not expect the provisions of ASU 2017-12 to have a material impact on our consolidated financial statements. Use of Estimates The preparation of these consolidated financial statements in conformity with GAAP requires our management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingencies at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. We evaluate our estimates, including those related to assessing the carrying values of our real estate properties, goodwill, accrued liabilities and performance-based equity compensation plans. We base our estimates on historical experience, current market conditions, and various other assumptions that we believe to be reasonable under the circumstances. Actual results may vary from those estimates and those estimates could vary under different assumptions or conditions. Reclassifications Certain immaterial amounts included in the consolidated financial statements for 2016 and 2015 have been reclassified to conform to the 2017 financial statement presentation. Investments in Real Estate Real estate investments are carried at cost less accumulated depreciation and amortization. The cost of real estate includes the purchase price of property and leasehold improvements. Expenditures for maintenance and repairs are expensed as incurred. Significant renovations and betterments that extend the economic useful lives of assets are capitalized. During land development and construction periods, we capitalize construction costs, legal fees, financing costs, real estate taxes and insurance, rent expense and internal costs of personnel performing development, if such costs are incremental and identifiable to a specific development project. Capitalization of costs begins upon commencement of development efforts and ceases when the project is ready for its intended use and held available for occupancy. Interest is capitalized during the period of development based upon applying the weighted-average borrowing rate to the actual development costs expended. Capitalized interest costs were $3.3 million, $4.1 million and $3.7 million for the years ended December 31, 2017, 2016, and 2015, respectively. Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: Buildings 27 to 40 years Building improvements 1 to 10 years Leasehold improvements The shorter of the lease term or useful life of the asset Depreciation expense was $108.1 million, $89.2 million and $76.1 million for the years ended December 31, 2017, 2016 and 2015, respectively. Acquisition of Investment in Real Estate When accounting for business combinations and asset acquisitions, the fair value of the real estate acquired is allocated to the acquired tangible assets, consisting primarily of land, building and building improvements, and identified intangible assets and liabilities, consisting of the value of above-market and below-market leases, value of in-place leases and the value of customer relationships. The fair value of the land and building of an acquired property is determined by valuing the property as if it were vacant, and the “as-if-vacant” fair value is then allocated to land and building based on management’s determination of the fair values of these assets. Management determines the as-if-vacant fair value of a property using methods similar to those used by independent appraisers. Factors considered by management in performing these analyses include an estimate of carrying costs during the expected lease-up periods considering current market conditions and costs to execute similar leases. The fair value of intangibles related to in-place leases includes the value of lease intangibles for above-market and below-market leases, lease origination costs, and customer relationships, determined on a lease-by-lease basis. Above-market and below-market leases are valued based on the present value (using an interest rate which reflects the risks associated with the leases acquired) of the difference between (i) the contractual amounts to be paid pursuant to the in-place leases and (ii) management's estimate of market lease rates for the corresponding in-place leases, measured over a period equal to the remaining noncancelable term of the lease and, for below-market leases, over a time period equal to the initial term plus any below-market fixed rate renewal periods. Lease origination costs include estimates of costs avoided associated with leasing the property, including tenant allowances and improvements and leasing commissions. Customer relationship intangibles relate to the additional revenue opportunities expected to be generated through interconnection services and utility services to be provided to the in-place lease tenants. The capitalized values for above and below-market lease intangibles, lease origination costs, and customer relationships are amortized over the term of the underlying leases or the expected customer relationship. Amortization related to above-market and below-market leases where the Company is the lessor is recorded as either a reduction of or an increase to rental revenue, amortization related to above-market and below-market leases where the Company is the lessee is recorded as either a reduction of or an increase to rent expense. If a lease is terminated prior to its stated expiration, all unamortized amounts relating to that lease are written off. The carrying value of intangible assets is reviewed for impairment in connection with its respective asset group whenever events or changes in circumstances indicate that the asset group may not be recoverable. An impairment loss is recognized if the carrying amount of the asset group is not recoverable and its carrying amount exceeds its estimated fair value. No impairment loss related to these intangible assets was recognized for the years ended December 31, 2017, 2016, or 2015. The excess of the cost of an acquired business over the net of the amounts assigned to assets acquired (including identified intangible assets) and liabilities assumed is recorded as goodwill. As of December 31, 2017, and 2016, we had approximately $40.6 million and $41.2 million of goodwill, respectively. The Company’s goodwill has an indeterminate life and is not amortized, but is tested for impairment on an annual basis, or more frequently if events or changes in circumstances indicate that the asset might be impaired. No impairment loss was recognized for the years ended December 31, 2017, 2016, or 2015. During the year ended December 31, 2017, goodwill was reduced by $0.5 million due to the receipt of cash from an escrow account associated with an entity acquired at the time of the Company’s initial public offering (“IPO”). Cash and Cash Equivalents Cash and cash equivalents include all non‑restricted cash held in financial institutions and other non‑restricted highly liquid short‑term investments with original maturities at acquisition of three months or less. Deferred Costs Deferred leasing costs include commissions paid to third parties, including brokers, leasing and referral agents, and internal sales commissions paid to employees for successful execution of lease agreements. These commissions and other direct and incremental costs incurred to obtain new customer leases are capitalized and amortized over the term of the related leases using the straight-line method. If a customer lease terminates prior to the expiration of its initial term, any unamortized deferred costs related to the lease are written off to amortization expense. Deferred leasing costs are included within other assets in the consolidated balance sheets and consisted of the following, net of amortization, as of December 31, 2017, and 2016 (in thousands): December 31, December 31, 2017 2016 Internal sales commissions $ 17,402 $ 18,748 Third party commissions 11,802 13,643 External legal counsel 775 730 Total $ 29,979 $ 33,121 Deferred financing costs include costs incurred in connection with obtaining debt and extending existing debt. These financing costs are capitalized and amortized on a straight-line basis, which approximates the effective-interest method, over the term of the loan and the amortization is included as a component of interest expense. Depending on the type of debt instrument, deferred financing costs are reported either in other assets or as a direct deduction from the carrying amount of the related debt liabilities in our consolidated balance sheets. Recoverability of Long‑Lived Assets We review our long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Impairment is recognized when estimated expected future cash flows (undiscounted and without interest charges) are less than the carrying amount of the assets. The estimation of expected future net cash flows is inherently uncertain and relies, to a considerable extent, on assumptions regarding current and future economics and market conditions and the availability of capital. If, in future periods, there are changes in the estimates or assumptions incorporated into the impairment review analysis, the changes could result in an adjustment to the carrying amount of the long-lived assets. To the extent that impairment has occurred, the excess of the carrying amount of long-lived assets over its estimated fair value would be recognized as an impairment loss charged to net income. For the years ended December 31, 2017, 2016, and 2015, no impairment of long-lived assets was recognized. Derivative Instruments and Hedging Activities We reflect all derivative instruments at fair value as either assets or liabilities on the consolidated balance sheets. For those derivative instruments that are designated and qualify as hedging instruments, we record the effective portion of the gain or loss on the hedging instruments as a component of accumulated other comprehensive income or loss. Any ineffective portion of a derivative’s change in fair value is immediately recognized within net income. For derivatives that do not meet the criteria for hedge accounting, changes in fair value are immediately recognized within net income. See additional discussion in Note 9 Derivatives and Hedging Activities. Internal-Use Software We recognize internal-use software development costs based on the development stage of the project and nature of the cost. Internal and external costs incurred during the preliminary project stage are expensed as they are incurred. Internal and external costs incurred to develop internal-use software during the application development stage are capitalized. Internal and external training costs and maintenance costs during the post-implementation-operation stage are expensed as incurred. Completed projects are placed into service and amortized over the estimated useful life of the software. For the years ended December 31, 2017, and 2016, no impairment was recognized related to impairment of internal use software. For the year ended December 31, 2015, we recorded $0.3 million related to impairment of internal use software. Our SaaS arrangements are capitalized when we have a contractual right to take possession of the software at any point during the period without significant penalty and it is feasible to run the software on our own hardware or we are able to contract with an unrelated third party to run the software. Any arrangements that do not meet these criterion are accounted for as a service contract and the costs are expensed as the services are performed. Revenue Recognition Our customer arrangements contain lease and nonlease elements. Consideration called for by the arrangement is separated at the inception of the arrangement into the lease and nonlease elements based on the relative fair value of each element. For the lease elements, rental revenue is recognized on a straight‑line basis over the customer’s lease term. The excess of rents recognized over amounts contractually due pursuant to the underlying leases is recorded as deferred rent receivable within our consolidated balance sheets. Nonlease elements include power and interconnection services. Power revenue is recognized each month as the power services are delivered to and utilized by our customers. Power services may include non‑recurring customer set‑up charges and installation fees that are initially deferred and recognized over the expected performance period. Interconnection services are contracted on a month-to-month basis and revenue is recognized each month as these services are delivered to and utilized by our customers. Some of our data center leases contain provisions under which our customers reimburse us for a portion of real estate taxes, insurance, common area maintenance, and other recoverable costs. Such customer reimbursements are recognized in the period that the expenses are recognized. A provision for uncollectible accounts is recorded if a receivable balance relating to contractual rent, rent recorded on a straight-line basis, tenant reimbursements or other billed amounts is considered by management to be uncollectible. At December 31, 2017, and 2016, the allowance for doubtful accounts totaled $1.1 million and $0.2 million, respectively. Share‑Based Compensation We account for share-based compensation using the fair value method of accounting. The estimated fair value of the stock options granted by us is calculated based on the Black-Scholes option-pricing model. The fair value of restricted share-based and Operating Partnership unit compensation is based on the fair value of our common stock on the date of the grant. The fair value of performance share awards, which have a market condition, is based on a Monte Carlo simulation. The fair value for all share-based compensation is amortized on a straight-line basis over the vesting period. We have elected to account for forfeitures as they occur. Asset Retirement and Environmental Remediation Obligations We record accruals for estimated asset retirement and environmental remediation obligations. The obligations relate primarily to the removal of asbestos during development of properties as well as the estimated equipment removal costs upon termination of a certain lease where we are the lessee. At December 31, 2017, and 2016, the amount included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.5 million and $1.4 million, respectively. Income Taxes We elected to be taxed as a REIT under the Internal Revenue Code of 1986, as amended (the “Code”), commencing with our taxable year ended December 31, 2010. To qualify as a REIT, we are required to distribute at least 90% of our taxable income to our stockholders and meet various other requirements imposed by the Code relating to such matters as operating results, asset holdings, distribution levels and diversity of stock ownership. Provided we qualify for taxation as a REIT, we generally are not subject to corporate level federal income tax on the earnings distributed currently to our stockholders. If we fail to qualify as a REIT in any taxable year, and are unable to avail ourselves of certain savings provisions set forth in the Code, all of our taxable income would be subject to federal income tax at regular corporate rates, including any applicable alternative minimum tax. To maintain REIT status, we must distribute a minimum of 90% of our taxable income. However, it is our policy and intent, subject to change, to distribute 100% of our taxable income and therefore, no provision is required in the accompanying consolidated financial statements for federal income taxes with regards to activities of the REIT and its subsidiary pass‑through entities. The allocable share of taxable income is included in the income tax returns of its stockholders. The Company is subject to the statutory requirements of the locations in which it conducts business. State and local income taxes are accrued as deemed required in the best judgment of management based on analysis and interpretation of respective tax laws. We have elected to treat certain subsidiaries as taxable REIT subsidiaries (“TRS”). Certain activities that we undertake must be conducted by a TRS, such as services for our tenants that could be considered otherwise impermissible for us to perform and holding assets that we cannot hold directly. A TRS is subject to corporate level federal and state income taxes. Deferred income taxes are recognized in certain taxable entities. Deferred income tax generally is a function of the period’s temporary differences (items that are treated differently for tax purposes than for financial reporting purposes), the utilization of tax net operating losses generated in prior years that previously had been recognized as deferred income tax assets and the reversal of any previously recorded deferred income tax liabilities. A valuation allowance for deferred income tax assets is provided if we believe all or some portion of the deferred income tax asset may more likely than not be realized. Any increase or decrease in the valuation allowance resulting from a change in circumstances that causes a change in the estimated realizability of the related deferred income tax asset is included in deferred tax expense. As of December 31, 2017, and 2016, the gross deferred income taxes were not material. We currently have no liabilities for uncertain income tax positions. The earliest tax year for which we are subject to examination is 2014. Concentration of Credit Risks Our cash and cash equivalents are maintained in various financial institutions, which, at times, may exceed federally insured limits. We have not experienced any losses in such accounts, and management believes that the Company is not exposed to any significant credit risk in this area. We have no off‑balance sheet concentrations of credit risk, such as foreign exchange contracts, option contracts, or foreign currency hedging arrangements. Segment Information We manage our business as one reportable segment consisting of investments in data centers located in the United States. Although we provide services in several markets, these operations have been aggregated into one reportable segment based on the similar economic characteristics amongst all markets, including the nature of the services provided and the type of customers purchasing these services.

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2017
Acquisitions
Acquisitions	3. Acquisitions On November 10, 2016, we paid cash consideration of $65.0 million to acquire a 21.75-acre operating light-industrial / flex office park located in Reston, Virginia, referred to as the Reston Campus Expansion. The transaction was accounted for as an asset acquisition. We plan to redevelop the existing buildings into over 611,000 of incremental data center NRSF across multiple phases as existing customers vacate their existing space upon expiration of their leases. The acquisition and redevelopment of the Reston Campus Expansion will extend our position as a multi-tenant data center provider in the Northern Virginia market. On August 29, 2017, we acquired a two-acre land parcel adjacent to our existing Santa Clara campus, with a total real estate cost of $12.2 million. In accordance with the purchase and sale agreement, the seller has committed to pay us $0.3 million over six months to assist them in vacating the property. We have accounted for the $0.3 million of cash receipts as ancillary operations which reduces the basis of the land parcel. We plan to build a turn-key data center on the acquired land parcel, which we refer to as SV8, upon the receipt of necessary entitlements. On January 29, 2018, we acquired a two-acre land parcel located in downtown Chicago, Illinois, for a purchase price of $4.5 million. We expect to build a 175,000 square foot turn-key data center building on the acquired land parcel, which we refer to as CH2, upon the receipt of necessary entitlements.

Acquired Intangible Assets and

Acquired Intangible Assets and Liabilities	12 Months Ended
	Dec. 31, 2017
Acquired Intangible Assets and Liabilities
Acquired Intangible Assets and Liabilities	4. Acquired Intangible Assets and Liabilities We did not add new intangible assets or liabilities during the year ended December 31, 2017. During the year ended December 31, 2016, through the acquisition of the Reston Campus Expansion, we added $7.5 million of new intangible assets and $0.4 million of new intangible liabilities with a weighted average life of 4.3 years and 5.7 years, respectively. Above-market and below-market lease intangibles and other intangibles balances at December 31, 2017, and 2016, are as follows (in thousands): December 31, December 31, 2017 2016 Lease contracts above-market value $ 714 $ 2,080 Accumulated amortization (532) (1,698) Lease contracts above-market value, net $ 182 $ 382 Lease contracts below-market value $ 9,102 $ 9,731 Accumulated amortization (5,608) (5,439) Lease contracts below-market value, net $ 3,494 $ 4,292 Other identified intangible assets $ 14,174 $ 20,218 Accumulated amortization (8,053) (10,687) Other identified intangible assets, net $ 6,121 $ 9,531 The net effect of amortization of acquired above‑market and below‑market leases resulted in an increase to rental revenue of $0.6 million, $0.5 million, and $0.5 million, for the years ended December 31, 2017, 2016, and 2015, respectively. The estimated amortization of acquired below‑market leases, net of acquired above‑market leases for each of the five succeeding fiscal years, which will be a net increase to rental revenue, is as follows (in thousands): Year Ending December 31, 2018 $ 456 2019 236 2020 241 2021 234 2022 208 Thereafter 1,937 Total $ 3,312 Amortization of all other identified intangible assets was $3.4 million, $2.0 million, and $2.0 million, for the years ended December 31, 2017, 2016, and 2015, respectively. The estimated amortization of all other identified intangible assets for each of the five succeeding fiscal years is as follows (in thousands): Year Ending December 31, 2018 $ 1,931 2019 1,481 2020 897 2021 856 2022 705 Thereafter 251 Total $ 6,121

Investment in Real Estate

Investment in Real Estate	12 Months Ended
	Dec. 31, 2017
Investment in Real Estate
Investment in Real Estate	5. Investment in Real Estate The following is a summary of the properties owned or leased by market at December 31, 2017 (in thousands): Buildings and Construction in Market Land Improvements Progress Total Cost Boston $ 5,154 $ 106,317 $ 28 $ 111,499 Chicago 5,493 101,798 10,470 117,761 Denver — 19,193 187 19,380 Los Angeles 28,467 273,000 44,503 345,970 Miami 728 11,612 28 12,368 New York 2,388 132,413 41,195 175,996 Northern Virginia (1) 23,642 300,329 50,336 374,307 San Francisco Bay (2) 31,386 616,394 16,156 663,936 Total $ 97,258 $ 1,561,056 $ 162,903 $ 1,821,217 (1) On February 23, 2017, we executed a ten-year lease for 25,000 square feet at a new property, which we refer to as DC2, to further expand our data center presence in Washington D.C., within our Northern Virginia market. DC2 is under construction as of December 31, 2017. (2) On August 29, 2017, we acquired a two-acre land parcel adjacent to our existing Santa Clara campus, with a total real estate cost of $12.2 million. In accordance with the purchase and sale agreement, the seller has committed to pay us $0.3 million over six months to assist them in vacating the property. We will account for the $0.3 million of cash receipts as ancillary operations which will reduce the basis of the land parcel. We plan to build a turn-key data center on the acquired land parcel, which we refer to as SV8, upon the receipt of necessary entitlements.

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2017
Other Assets.
Other Assets	6. Other Assets Our other assets consisted of the following, net of amortization and depreciation, if applicable, as of December 31, 2017, and 2016 (in thousands): December 31, December 31, 2017 2016 Deferred rent receivable $ 40,038 $ 36,393 Deferred leasing costs 29,979 33,121 Internal-use software 17,477 14,440 Prepaid expenses 6,770 4,390 Corporate furniture, fixtures and equipment 6,408 5,356 Deferred financing costs - revolving credit facility 957 1,604 Other 1,872 1,068 Total $ 103,501 $ 96,372 Deferred leasing costs are amortized as amortization expense on a straight‑line basis over the remaining lease terms of the underlying leases. The estimated amortization of deferred leasing costs for each of the five succeeding fiscal years is as follows (in thousands): Year Ending December 31, 2018 $ 13,845 2019 8,194 2020 3,900 2021 1,560 2022 1,019 Thereafter 1,461 Total $ 29,979

Leases

Leases	12 Months Ended
	Dec. 31, 2017
Leases
Leases	7. Leases As a lessor, the Company leases data center space and office and light-industrial space under noncancelable operating lease agreements. The future minimum lease payments to be received under noncancelable operating leases in effect at December 31, 2017, are as follows (in thousands): Year Ending December 31, 2018 $ 237,706 2019 174,763 2020 120,834 2021 92,684 2022 68,259 Thereafter 161,525 Total $ 855,771 As the lessee, the Company currently leases data center space under noncancelable operating lease agreements at NY1, LA1, DC1, DC2, DE1, and DE2, and its headquarters located in Denver, CO. The future minimum lease payments to be paid under noncancelable real estate leases in effect at December 31, 2017, are as follows (in thousands): Year Ending December 31, 2018 $ 21,264 2019 22,491 2020 22,928 2021 22,676 2022 15,550 Thereafter 11,874 Total $ 116,783

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt
Debt	8. Debt A summary of outstanding indebtedness as of December 31, 2017, and 2016, is as follows (in thousands): Maturity December 31, December 31, Interest Rate Date 2017 2016 Revolving credit facility 3.11% and 2.32% at December 31, 2017, and December 31, 2016, respectively June 24, 2019 $ 169,500 $ 194,000 2020 Senior unsecured term loan (1) 3.00% and 2.60% at December 31, 2017, and December 31, 2016, respectively June 24, 2020 150,000 150,000 2021 Senior unsecured term loan 3.06% and 2.27% at December 31, 2017, and December 31, 2016, respectively February 2, 2021 100,000 100,000 2022 Senior unsecured term loan (2) 3.04% and 3.23% at December 31, 2017, and December 31, 2016, respectively April 19, 2022 200,000 100,000 2023 Senior unsecured notes 4.19% at December 31, 2017, and December 31, 2016, respectively June 15, 2023 150,000 150,000 2024 Senior unsecured notes 3.91% at December 31, 2017 April 20, 2024 175,000 — Total principal outstanding ` 944,500 694,000 Unamortized deferred financing costs (4,930) (3,550) Total debt $ 939,570 $ 690,450 (1) Our Operating Partnership has in place a swap agreement with respect to the 2020 Term Loan (as defined below) to swap the variable interest rate associated with $75 million, or 50% of the principal amount, of the 2020 Term Loan to a fixed rate of approximately 2.93% per annum at our current leverage ratio. The interest rate on the remaining $75 million of the 2020 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of December 31, 2017, is 3.00%. See “Derivatives and Hedging Activities” in Note 9 below. (2) Our Operating Partnership has in place a swap agreement with respect to the 2022 Term Loan (as defined below) to swap the variable interest rate associated with $50 million, or 25% of the principal amount, of the 2022 Term Loan to a fixed rate of approximately 2.98% per annum at our current leverage ratio through January 31, 2019. The interest rate on the remaining $150 million of the 2022 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of December 31, 2017, is 3.04%. See “Derivatives and Hedging Activities” in Note 9 below. Revolving Credit Facility On February 2, 2016, our Operating Partnership and certain subsidiary co-borrowers entered into the first amendment to the third amended and restated credit agreement (as amended, the “Credit Agreement”) with a group of lenders for which KeyBank National Association acts as the administrative agent. The Credit Agreement maturity date is June 24, 2019, with a one-time extension option, which, if exercised, would extend the maturity date to June 24, 2020. The exercise of the extension option is subject to the payment of an extension fee equal to 10 basis points of the total commitment under the Credit Agreement at initial maturity and certain other customary conditions. The Credit Agreement includes a total commitment of $600 million, providing for a $350 million revolving credit facility, a $150 million unsecured term loan scheduled to mature on June 24, 2020, and a $100 million unsecured term loan scheduled to mature on February 2, 2021. See “2020 Senior Unsecured Term Loan” and “2021 Senior Unsecured Term Loan” below for a discussion of the $150 million and $100 million term loans. The Credit Agreement contains an accordion feature, which allows our Operating Partnership to increase the total commitment from $600 million to $800 million, under specified circumstances, including securing capital from new or existing lenders. Borrowings under the revolving credit facility bear interest at a variable rate per annum equal to either (i) LIBOR plus 155 basis points to 225 basis points, or (ii) a base rate plus 55 basis points to 125 basis points, each depending on our Operating Partnership's leverage ratio. At December 31, 2017, our Operating Partnership’s leverage ratio was 26.7% and the interest rate was LIBOR plus 155 basis points. The total amount available for borrowing under the revolving credit facility, which is part of the Credit Agreement, is equal to the lesser of $350.0 million or the availability calculated based on our unencumbered asset pool. As of December 31, 2017, the borrowing capacity was $350.0 million. As of December 31, 2017, $169.5 million was borrowed and outstanding, $4.9 million was outstanding under letters of credit and therefore $175.6 million remained available for us to borrow under the revolving credit facility. Our ability to borrow under the Credit Agreement is subject to ongoing compliance with a number of financial covenants and other customary restrictive covenants, including, among others: · a maximum leverage ratio (defined as total consolidated indebtedness to total gross asset value) of 60%, which, as of December 31, 2017, was 26.7%. · a maximum secured debt ratio (defined as total consolidated secured debt to total gross asset value) of 40%, which, as of December 31, 2017, was 0.0%. · a minimum fixed charge coverage ratio (defined as adjusted consolidated earnings before interest, taxes, depreciation and amortization to consolidated fixed charges) of 1.7 to 1.0, which, as of December 31, 2017, was 6.5 to 1.0; and · a maximum unhedged variable rate debt ratio (defined as unhedged variable rate indebtedness to gross asset value) of 30%, which, as of December 31, 2017, our unhedged variable rate debt ratio was 13.9%. The Credit Agreement ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Notes, and the 2024 Notes and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2017, we were in compliance with all of the financial covenants under the Credit Agreement. 2020 Senior Unsecured Term Loan On June 24, 2015, in connection with, and pursuant to the terms of, the Credit Agreement, our Operating Partnership and certain subsidiaries entered into a $150 million senior unsecured term loan (the "2020 Term Loan"). The 2020 Term Loan has a five-year term maturing on June 24, 2020. The 2020 Term Loan ranks pari passu with the 2021 Term Loan, the 2022 Term Loan, the 2023 Notes, the 2024 Notes and the Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2017, we were in compliance with all of the financial covenants under the 2020 Term Loan. The borrowings under the 2020 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 150 basis points to 220 basis points, or (ii) a base rate plus 50 basis points to 120 basis points, each depending on our Operating Partnership' s leverage ratio. At December 31, 2017, the Operating Partnership's leverage ratio was 26.7% and the interest rate was LIBOR plus 150 basis points. 2021 Senior Unsecured Term Loan On February 2, 2016, pursuant to the terms of the Credit Agreement, we partially exercised the accordion feature and entered into a $100 million senior unsecured term loan (the “2021 Term Loan”). The 2021 Term Loan has a five-year term maturing on February 2, 2021. The 2021 Term Loan ranks pari passu with the 2020 Term Loan, the 2022 Term Loan, the 2023 Notes, the 2024 Notes and the Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2017, we were in compliance with all of the financial covenants under the 2021 Term Loan. The borrowings under the 2021 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 150 basis points to 220 basis points, or (ii) a base rate plus 50 basis points to 120 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2017, our Operating Partnership’s leverage ratio was 26.7% and the interest rate was LIBOR plus 150 basis points. 2022 Senior Unsecured Term Loan On January 31, 2014, our Operating Partnership and certain subsidiaries entered into a $100 million senior unsecured term loan (the “2019 Term Loan”). The 2019 Term Loan had a five-year term and contained an accordion feature, which allowed our Operating Partnership to increase the total commitments by $100 million, to $200 million under specified circumstances including securing capital from new or existing lenders. On April 19, 2017, our Operating Partnership and certain subsidiaries amended and restated the 2019 Term Loan (as amended, the “2022 Term Loan”), to (i) exercise the accordion feature to increase the total commitments to $200 million, (ii) extend the maturity of the term loan from January 31, 2019, to April 19, 2022, (iii) amend the accordion feature to allow an increase in total commitments from $200 million to $300 million, under specified circumstances, including securing capital from new or existing lenders, and (iv) explicitly permit the issuance of the 2024 Notes defined below. The 2022 Term Loan ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2023 Notes, the 2024 Notes and the Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2017, we were in compliance with all of the financial covenants under the 2022 Term Loan. The borrowings under the 2022 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 150 basis points to 210 basis points, or (ii) a base rate plus 50 basis points to 110 basis points, each depending on our Operating Partnership's leverage ratio. At December 31, 2017, our Operating Partnership’s leverage ratio was 26.7% and the interest rate was LIBOR plus 150 basis points. 2023 Senior Unsecured Notes On June 15, 2016, our Operating Partnership issued an aggregate principal amount of $150 million, 4.19% senior unsecured notes due June 15, 2023 (the “2023 Notes”), in a private placement to certain accredited investors. The terms of the 2023 Notes are governed by a note purchase agreement, dated June 15, 2016 (the “2023 Note Purchase Agreement”), by and among our Operating Partnership, the Company and the purchasers of the 2023 Notes. Interest is payable semiannually, on the 15 th day of June and December of each year, commencing on December 15, 2016. The 2023 Notes are senior unsecured obligations of our Operating Partnership and are jointly and severally guaranteed by the Company and each of our Operating Partnership’s subsidiaries that guarantees indebtedness under our Operating Partnership’s Credit Agreement (the “Subsidiary Guarantors”). Our Operating Partnership may prepay all or a portion of the 2023 Notes upon notice to the holders for 100% of the principal amount so prepaid plus a make-whole premium as set forth in the 2023 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2023 Notes have the right to require our Operating Partnership to purchase 100% of such holder’s 2023 Notes in cash at a purchase price equal to 100% of the principal amount thereof plus accrued and unpaid interest, if any, to the date of repurchase. The 2023 Notes rank pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2024 Notes and the Credit Agreement. The 2023 Note Purchase Agreement contains the same financial covenants as the Credit Agreement, as described above. In addition, certain additional financial covenants in the Credit Agreement were automatically incorporated into the 2023 Note Purchase Agreement, and, subject to certain conditions, these additional financial covenants will be deleted, removed, amended or otherwise modified to be more or less restrictive if the analogous covenant in the Credit Agreement is so deleted, removed, amended or otherwise modified. These covenants are subject to a number of exceptions and qualifications set forth in the 2023 Note Purchase Agreement. As of December 31, 2017, we were in compliance with all of the financial covenants under the 2023 Note Purchase Agreement. 2024 Senior Unsecured Notes On April 20, 2017, our Operating Partnership issued an aggregate principal amount of $175 million, 3.91% senior unsecured notes due April 20, 2024 (the “2024 Notes”), in a private placement to certain accredited investors. The terms of the 2024 Notes are governed by a note purchase agreement, dated April 20, 2017 (the “2024 Note Purchase Agreement”), by and among our Operating Partnership, the Company and the purchasers of the 2024 Notes. Interest is payable semiannually, on the 15 th day of June and December of each year, commencing on December 15, 2017. The 2024 Notes are senior unsecured obligations of our Operating Partnership and are jointly and severally guaranteed by the Company and each of the Subsidiary Guarantors. Our Operating Partnership may prepay all or a portion of the 2024 Notes upon notice to the holders for 100% of the principal amount so prepaid plus a make-whole premium as set forth in the 2024 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2024 Notes will have the right to require our Operating Partnership to purchase 100% of such holders’ 2024 Notes in cash at a purchase price equal to 100% of the principal amount thereof plus accrued and unpaid interest, if any, to the date of repurchase. The 2024 Notes rank pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Notes and the Credit Agreement. The 2024 Note Purchase Agreement contains the same financial covenants as the Credit Agreement, as described above. In addition, certain additional financial covenants in the Credit Agreement were automatically incorporated into the 2024 Note Purchase Agreement, and, subject to certain conditions, these additional financial covenants will be deleted, removed, amended or otherwise modified to be more or less restrictive if the analogous covenant in the Credit Agreement is so deleted, removed, amended or otherwise modified. These covenants are subject to a number of exceptions and qualifications set forth in the 2024 Note Purchase Agreement. As of December 31, 2017, we were in compliance with all of the financial covenants under the 2024 Note Purchase Agreement. Debt Maturities The following table summarizes the amount of our outstanding debt as of December 31, 2017, based on when such debt currently becomes due (in thousands): Year Ending December 31, 2018 $ — 2019 169,500 2020 150,000 2021 100,000 2022 200,000 Thereafter 325,000 Total principal outstanding 944,500 Unamortized deferred financing costs (4,930) Total debt, net $ 939,570

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2017
Derivatives and Hedging Activities
Derivatives and Hedging Activities	9. Derivatives and Hedging Activities On April 21, 2017, we terminated $50 million of our previously existing $100 million five-year interest rate swap agreement that reduces our exposure to variability in cash flows relating to interest payments based on one-month LIBOR variable rate debt, resulting in a remaining $50 million interest rate swap effective through January 31, 2019, at approximately 2.98% per annum and 3.23% per annum for the years ending December 31, 2017, and 2016, respectively. On April 9, 2015, we entered into a $75 million forward starting five-year interest rate swap agreement, effective May 5, 2015, to protect against adverse fluctuation in interest rates. The swap reduces our exposure to variability in cash flows relating to interest payments on $75 million of one-month LIBOR variable rate debt and effectively fixes the interest rate at approximately 2.93% per annum. Both interest rate swap agreements were designated for hedge accounting. Risk Management Objective of Using Derivatives We are exposed to certain risks arising from both our business operations and economic conditions. We principally manage our exposures to a wide variety of business and operational risks through management of our core business activities. We manage economic risks, including interest rate, liquidity, and credit risk primarily by managing the amount, sources, and duration of our debt funding and the use of derivative financial instruments. Specifically, we enter into derivative financial instruments to manage exposures that arise from business activities that result in the receipt or payment of future known or uncertain cash amounts, the value of which are determined by interest rates. Our derivative financial instruments are used to manage differences in the amount, timing, and duration of our known or expected cash receipts and our known or expected cash payments principally related to our investments and borrowings. Cash Flow Hedges of Interest Rate Risk Our objectives in using interest rate derivatives are to reduce variability in interest expense and to manage our exposure to adverse interest rate movements. To accomplish this objective, we primarily use interest rate swaps as part of our interest rate risk management strategy. Interest rate swaps designated as cash flow hedges involve the receipt of variable amounts from a counterparty in exchange for the Company making fixed-rate payments over the life of the agreements without exchange of the underlying notional amount. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in accumulated other comprehensive income or loss on the consolidated balance sheets and is subsequently reclassified into earnings in the period that the hedged forecasted transaction affects earnings. The amounts recorded in other comprehensive income or loss related to the unrealized gain or loss on derivative contracts were a gain of $0.6 million and losses of $1.1 million, and $2.4 million for the years ended December 31, 2017, 2016, and 2015, respectively. The amounts reclassified from other comprehensive income or loss to interest expense on the consolidated statements of operations were $0.6 million, $1.7 million, and $1.9 million for the years ended December 31, 2017, 2016, and 2015, respectively. Any ineffective portion of the change in fair value of the derivatives is recognized directly in net income. During the years ended December 31, 2017, 2016, and 2015, we did not record any amount in net income related to derivatives as there was no hedge ineffectiveness. Amounts reported in accumulated other comprehensive income or loss related to derivatives will be reclassified to interest expense as interest payments are made on our variable-rate debt. During the subsequent twelve months, beginning January 1, 2018, we estimate that $0.3 million will be reclassified as an increase to interest expense. Derivatives are recorded at fair value in our consolidated balance sheets in other assets or unearned revenue, prepaid rent and other liabilities, as applicable. We do not net our derivative position by counterparty for purposes of balance sheet presentation and disclosure. We had zero and $0.4 million in derivative liabilities recognized in unearned revenue, prepaid rent and other liabilities in our consolidated balance sheets as of December 31, 2017 and 2016, respectively. We also had $1.1 million and $0.3 million in derivative assets recognized in other assets in our consolidated balance sheet as of December 31, 2017, and 2016, respectively.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity
Stockholders' Equity	10. Stockholders’ Equity At December 31, 2017, 120 million shares were authorized to be issued by the Company, of which 100 million shares represent common stock and 20 million represent preferred stock. The Board may, without stockholder approval, classify or reclassify any unissued shares of stock to increase or decrease the authorized number of shares of common stock and preferred stock and to establish the preferences and rights of any preferred stock or other class of series of shares to be issued. Common Stock At December 31, 2017, we had 34.2 million shares of common stock issued and outstanding. Since our IPO, we have issued common stock under our 2010 Equity Incentive Award Plan, in which certain of our employees and outside directors participate in stock‑based compensation plans that provide compensation in the form of common stock. Under the Equity Incentive Award Plan, we received proceeds (made payments) from the exercise of stock options, net of settlements, of $4.8 million, $1.3 million, and ($2.9) million for the years ended December 31, 2017, 2016, and 2015, respectively. Also, we issued shares of common stock as restricted stock award and performance stock award grants, net of forfeitures, and option exercises, net of settlements, in the amount of 329,033, 239,068 and 393,337 during the years ended December 31, 2017, 2016, and 2015, respectively. See Note 12, Equity Incentive Award Plan, for additional information on our 2010 Equity Incentive Award Plan. During the year ended December 31, 2017, and 2016, 15,011 and 7,000 common Operating Partnership units held by other third parties were redeemed for shares of our common stock, respectively. During the year ended December 31, 2016, and 2015, 3,000,000 and 8,500,000 common Operating Partnership units held by the Carlyle Group and other third parties were redeemed for shares of our common stock in connection with the offer and sale of 3,000,000 and 8,500,000 shares of our common stock, respectively. See Note 11, Noncontrolling Interests – Operating Partnership, for additional information. Preferred Stock On December 12, 2012, the Company issued an aggregate of 4,600,000 shares of its 7.25% Series A Cumulative Redeemable Preferred Stock, or the Series A Preferred Stock, for net proceeds of $110.6 million. Dividends were cumulative on the Series A Preferred Stock from the date of original issuance in the amount of $1.8125 per share each year, which was equivalent to 7.25% of the $25.00 liquidation preference per share. Dividends on the Series A Preferred Stock were payable quarterly in arrears. On December 12, 2017, the Company redeemed an aggregate of 4,600,000 shares of its Series A Preferred Stock for $25.00 per share, plus all accrued and unpaid dividends in an amount equal to $0.292014 per share, for a total payment of $25.292014 per share, or $116.3 million in aggregate. The excess of the redemption price over the carrying value of the Series A Preferred Stock of approximately $4.3 million relates to the original issuance costs and was recorded as a reduction to net income attributable to common shares. Dividends In 2017, 2016, and 2015, we paid all our dividends in cash. The following summarizes the taxability of our common and preferred stock dividends per share for the years then ended: Year Ended December 31, Record Date 2017 2016 2015 Common Stock: Ordinary income $ 2.13 $ 2.36 $ 1.70 Qualified dividend — — — Capital gains — — — Return of capital 0.91 — — Total dividend (1) $ 3.04 $ 2.36 $ 1.70 Preferred Stock: Ordinary income $ 1.36 $ 1.81 $ 1.81 Qualified dividend — — — Capital gains — — — Total dividend (2) $ 1.36 $ 1.81 $ 1.81 (1) A $0.98 per share or unit distribution was paid in January, all of which will apply to tax year 2018. Of the $2.36 taxable dividend in 2016, $0.80 was paid in January 2017, of which $0.44 will apply to tax year 2017. Of the $1.70 taxable dividend in 2015, $0.53 was paid in January 2016, of which $0.41 will apply to tax year 2016. (2) All outstanding shares and dividends of the 7.25% Series A Cumulative Redeemable Preferred Stock were redeemed at $25.292014 per share at December 12, 2017. Of the $1.81 taxable dividend in 2016, $0.45 was paid in January 2017. Of the $1.81 taxable dividend in 2015, $0.45 was paid in January 2016. In order to comply with the real estate investment trust requirements of the Code, we are generally required to make common stock distributions (other than capital gain distributions) to our stockholders at least equal to (i) the sum of (a) 90% of our “REIT taxable income” computed without regard to the dividends paid deduction and net capital gains and (b) 90% of the net taxable income (after tax), if any, from foreclosure property, minus (ii) certain excess non‑cash income. Our common stock dividend policy is to distribute, at a minimum, a percentage of our cash flow to ensure we will meet the distribution requirements of the Code, while allowing us to retain cash to meet other needs, such as capital improvements and other investment activities. Any subsequent increases and/or anticipated increases are correlated to increases in our growth of cash flow. Common stock dividends are characterized for federal income tax purposes as ordinary income, qualified dividend, capital gains, non‑taxable return of capital or a combination of the four. Common stock dividends that exceed our current and accumulated earnings and profits (calculated for tax purposes) constitute a return of capital rather than a dividend and generally reduce the stockholder’s basis in the common stock. To the extent that a dividend exceeds both current and accumulated earnings and profits and the stockholder’s basis in the common stock, it will generally be treated as a gain from the sale or exchange of that stockholder’s common stock. At the beginning of each year, we notify our stockholders of the taxability of the common stock dividends paid during the preceding year. Pursuant to the terms of our preferred stock, we were restricted from declaring or paying any dividend with respect to our common stock unless and until all cumulative dividends with respect to the preferred stock had been paid and sufficient funds had been set aside for dividends that had been declared for the relevant dividend period with respect to the preferred stock. On December 12, 2017, we redeemed our preferred stock in its entirety. Our federal tax return for the year ended December 31, 2017 has not been filed. The taxability information presented for our dividends paid in 2017 is based upon management’s estimates.

Noncontrolling Interests - Oper

Noncontrolling Interests - Operating Partnership	12 Months Ended
	Dec. 31, 2017
Noncontrolling Interests - Operating Partnership
Noncontrolling Interests - Operating Partnership	11. Noncontrolling Interests — Operating Partnership Noncontrolling interests represent the limited partnership interests in our Operating Partnership held by individuals and entities other than CoreSite Realty Corporation. The current holders of common Operating Partnership units are eligible to have the common Operating Partnership units redeemed for cash or common stock on a one‑for‑one basis, at our option. December 31, 2017 December 31, 2016 Number of Units Percentage of Total Number of Units Percentage of Total CoreSite Realty Corporation 33,826,264 71.0 % 33,376,568 70.7 % Noncontrolling interests 13,836,336 29.0 13,851,347 29.3 Total 47,662,600 100.0 % 47,227,915 100.0 % For each share of common stock issued by us, our Operating Partnership issues to us an equivalent common Operating Partnership unit. During the year ended December 31, 2017, we issued 449,696 shares of common stock related to employee compensation arrangements and therefore an equivalent number of common Operating Partnership units were issued to us by our Operating Partnership. Holders of common Operating Partnership units of record as of March 31, 2017, June 30, 2017, September 29, 2017, and December 29, 2017, received quarterly distributions of $0.80 per unit, $0.90 per unit, $0.90 per unit, and $0.98 per unit, respectively, payable in correlation with declared dividends on common stock. During the years ended December 31, 2017, 2016, and 2015, 15,011, 3,007,000, and 8,500,000 common Operating Partnership units held by The Carlyle Group and other third parties were redeemed for shares of our common stock in connection with the sale of 15,011, 3,007,000, and 8,500,000 shares of our common stock, respectively. These redemptions were recorded as $0.2 million, $37.4 million, and $110.2 million reductions to noncontrolling interests in our Operating Partnership and an increase to total stockholder’s equity in the consolidated balance sheets for the years ended December 31, 2017, 2016, and 2015, respectively. The redemption value of the noncontrolling interests at December 31, 2017, was $1.6 billion based on the closing price of the Company’s common stock of $113.90 on the last trading day prior to that date.

Equity Incentive Award Plan

Equity Incentive Award Plan	12 Months Ended
	Dec. 31, 2017
Equity Incentive Award Plan
Equity Incentive Plan	12. Equity Incentive Plan Our Board of Directors adopted and, with the approval of our stockholders, amended the 2010 Equity Incentive Plan (as amended, the “2010 Plan”). The 2010 Plan is administered by the Compensation Committee of our Board of Directors. Awards issuable under the 2010 Plan include common stock, stock options, restricted stock, stock appreciation rights, dividend equivalents, Operating Partnership units and other incentive awards. We have reserved a total of 6,000,000 shares of our common stock for issuance pursuant to the 2010 Plan, which may be adjusted for changes in our capitalization and certain corporate transactions. To the extent that an award expires, terminates or lapses, or an award is settled in cash without the delivery of shares of common stock to the participant, then any unvested shares subject to the award will be available for future grant or sale under the 2010 Plan. Shares of restricted stock that are forfeited or repurchased by us pursuant to the 2010 Plan may again be awarded under the 2010 Plan. The payment of dividend equivalents in cash in conjunction with any outstanding awards will not be counted against the shares available for issuance under the 2010 Plan. As of December 31, 2017, 3,048,508 shares of our common stock were available for issuance pursuant to the 2010 Plan. Stock Options Stock option awards are granted with an exercise price equal to the closing market price of the Company’s common stock on the date of grant. The fair value of each option granted under the 2010 Plan is estimated on the date of grant using the Black‑Scholes option‑pricing model. The fair values are amortized on a straight‑line basis over the vesting periods. Stock options have not been granted since the year ending December 31, 2013. The following table sets forth stock option activity under the 2010 Plan for the years ended December 31, 2017, 2016, and 2015. Number of Weighted Weighted Aggregate Options outstanding, December 31, 2014 758,095 $ 20.94 6.7 years $ 13.7 million Granted — — Exercised (410,322) 20.15 $ 12.0 million Forfeited (7,281) 27.53 Expired (282) 15.23 Options outstanding, December 31, 2015 340,210 $ 21.77 5.8 years $ 11.9 million Granted — — Exercised (74,660) 17.27 $ 3.8 million Forfeited — — Expired — — Options outstanding, December 31, 2016 265,550 $ 22.96 5.0 years $ 15.0 million Granted — — Exercised (197,761) 24.37 $ 13.3 million Forfeited — — Expired — — Options outstanding, December 31, 2017 67,789 $ 19.12 3.4 years $ 6.4 million Exercisable at December 31, 2015 234,943 $ 18.47 5.4 years $ 9.0 million 2016 249,227 $ 22.29 4.9 years $ 14.2 million 2017 67,789 $ 19.12 3.4 years $ 6.4 million The following table sets forth the number of shares subject to options that are unvested as of December 31, 2017, 2016, and 2015, and the fair value of these options at the grant date: Number of Weighted Unvested balance, December 31, 2014 283,964 $ 7.75 Granted — — Forfeited (7,281) 8.48 Vested (171,416) 6.86 Unvested balance, December 31, 2015 105,267 $ 9.13 Granted — — Forfeited — — Vested (88,944) 8.97 Unvested balance, December 31, 2016 16,323 $ 9.99 Granted — — Forfeited — — Vested (16,323) 9.99 Unvested balance, December 31, 2017 — $ — As of December 31, 2017, all stock option awards are fully vested. As the Company has been a publicly traded company only since September 28, 2010, expected volatilities used in the Black-Scholes model are based on the historical volatility of a group of comparable REITs. We utilize the simplified method of estimating the term for options granted due to the lack of historical exercise data necessary to provide a reasonable basis upon which to estimate the term. The risk-free rate for periods within the contractual life of the option is based on the U.S. Treasury yield curve in effect at the time of grant. No options were granted during the years ended 2017, 2016 or 2015. Restricted Stock Awards and Units Restricted stock awards and restricted stock units, or RSUs, are granted with a fair value equal to the closing market price of the Company's common stock on the date of grant. The principal difference between restricted stock awards and RSUs is that RSUs are not shares of our common stock and do not have any of the rights or privileges thereof, including voting rights. On the applicable vesting date, the holder of an RSU becomes entitled to a share of common stock. The restricted stock awards and RSUs are amortized on a straight-line basis to expense over the vesting period. The following table sets forth the number of unvested restricted stock awards and RSUs and the weighted-average fair value of these awards at the date of grant: Restricted Stock Awards and Units Weighted Unvested balance, December 31, 2014 394,016 $ 29.10 Granted 187,644 47.41 Forfeited (34,788) 36.06 Vested (169,260) 26.96 Unvested balance, December 31, 2015 377,612 $ 38.51 Granted 182,628 66.69 Forfeited (37,383) 53.87 Vested (199,216) 36.57 Unvested balance, December 31, 2016 323,641 $ 53.84 Granted 139,788 85.79 Forfeited (34,020) 64.00 Vested (139,566) 44.31 Unvested balance, December 31, 2017 289,843 $ 70.71 As of December 31, 2017, total unearned compensation on restricted stock awards and RSUs was approximately $14.4 million, and the weighted-average vesting period was 2.4 years. Performance Stock Awards We grant long-term incentives to members of management in the form of performance-based restricted stock awards (“PSAs”) under the 2010 Plan. The number of PSAs earned is based on our achievement of relative total shareholder return (“TSR”) measured versus the MSCI US REIT Index over a three-year performance period and ranges between 25% and 175% of the target number of shares for PSAs granted in 2015, 2016, and 2017. The PSAs are granted at the maximum percentage of target and are retired annually to the extent we do not meet the maximum relative TSR performance threshold versus the MSCI US REIT Index. The PSAs are earned upon TSR achievement measured both annually and over the full three-year performance period. The PSAs have a service condition and will be released at the end of the three-year performance period, to the extent earned, provided that the holder continues to be employed or otherwise in service of the Company at the end of the three-year performance period. The PSAs are amortized on a straight-line basis to expense over the vesting period. Holders of the PSAs are entitled to dividends on the PSAs, which are accrued and paid in cash at the end of the three-year performance period. The following table sets forth the number of unvested PSAs and the weighted-average fair value of these awards at the date of grant: Weighted- Average Fair Performance-Based Restricted Stock Awards Value at Grant Minimum Maximum Target Date Unvested balance, December 31, 2014 — 85,851 57,233 $ 25.89 Granted 11,222 78,553 44,886 61.22 Performance adjustment (1) 9,981 (7,190) (1,465) — Forfeited (1,458) (13,657) (8,572) 43.38 Vested (594) (594) (594) 25.89 Unvested balance, December 31, 2015 19,151 142,963 91,488 $ 42.78 Granted 11,796 82,572 47,184 84.78 Performance adjustment (1) 29,256 — 11,893 — Forfeited (13,474) (55,916) (34,695) 69.12 Vested (1,894) (1,894) (1,894) 42.17 Unvested balance, December 31, 2016 44,835 167,725 113,976 $ 54.24 Granted 7,210 50,472 28,841 105.50 Performance adjustment (1) 62,841 — 23,725 — Forfeited (5,674) (16,836) (11,255) 79.09 Vested (70,518) (70,518) (70,518) 25.89 Unvested balance, December 31, 2017 38,694 130,843 84,769 $ 84.63 (1) Includes the annual adjustment for the number of PSAs earned based on our achievement of relative TSR measured versus the MSCI US REIT Index for the applicable performance periods. As of December 31, 2017, total unearned compensation on PSAs was approximately $3.3 million, and the weighted-average vesting period was 1.8 years. The fair value of each PSA award is estimated on the date of grant using a Monte Carlo simulation. The simulation requires assumptions for expected volatility, risk-free rate of return, and dividend yield. The following table summarizes the assumptions used to value the PSAs granted during the years ended December 31, 2017, 2016 and 2015: 2017 2016 2015 Expected term (in years) 2.81 2.83 2.83 Expected volatility 23.33 % 25.29 % 25.51 % Expected annual dividend (1) — — — Risk-free rate 1.60 % 0.97 % 1.02 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share
Earnings Per Share	13. Earnings Per Share Basic net income per share is calculated by dividing the net income attributable to common shares by the weighted-average number of common shares outstanding during the period. Diluted net income per share adjusts basic net income per share for the effects of potentially dilutive common shares, if the effect is not antidilutive. Potentially dilutive common stock consists of shares issuable under the 2010 Plan. The following is a summary of basic and diluted net income per share (in thousands, except share and per share amounts): Year Ended December 31, 2017 2016 2015 Net income attributable to common shares $ 62,605 $ 50,371 $ 26,368 Weighted-average common shares outstanding - basic 33,792,759 32,289,414 25,218,500 Effect of potentially dilutive common shares: Stock options 88,399 192,464 302,264 Unvested awards 177,791 250,181 185,804 Weighted-average common shares outstanding - diluted 34,058,949 32,732,059 25,706,568 Net income per share attributable to common shares Basic $ 1.85 $ 1.56 $ 1.05 Diluted $ 1.84 $ 1.54 $ 1.03 In the calculations above, we have excluded weighted-average potentially dilutive securities of 24,961, 29,618 and 117,048 for the years ended December 31, 2017, 2016, and 2015, respectively, as their effect would have been antidilutive.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2017
Employee Benefit Plan
Employee Benefit Plan	14. Employee Benefit Plan We have a tax qualified retirement 401(k) plan that provides employees with an opportunity to save for retirement on a tax advantaged basis. Employees are eligible to participate upon hire. Additionally, on the first of the month following after six months of employment, we provide a safe harbor contribution equal to 3% of the participant’s annual salary. The employee and employer contributions are limited to the maximum amount allowed by the Internal Revenue Service. Both employee and employer contributions vest immediately. For the years ended December 31, 2017, 2016, and 2015, our contributions were $1.1 million, $0.9 million, and $0.8 million, respectively.

Estimated Fair Value of Financi

Estimated Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2017
Estimated Fair Value of Financial Instruments
Estimated Fair Value of Financial Instruments	15. Estimated Fair Value of Financial Instruments Authoritative guidance issued by FASB establishes a hierarchy of valuation techniques based on the observability of inputs utilized in measuring assets and liabilities at fair values. This hierarchy establishes market‑based or observable inputs as the preferred source of values, followed by valuation models using management assumptions in the absence of market inputs. The three levels of the hierarchy under the authoritative guidance are as follows: Level 1 — Quoted prices (unadjusted) in active markets for identical assets or liabilities that the reporting entity can access at the assessment date. Level 2 — Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly. Level 3 — Unobservable inputs for the asset or liability. Our financial instruments consist of cash and cash equivalents, accounts and other receivables, interest rate swaps, the revolving credit facility, the senior unsecured term loans, senior unsecured notes, interest payable and accounts payable. The carrying values of cash and cash equivalents, accounts and other receivables, interest payable and accounts payable approximate fair values due to the short-term nature of these financial instruments. The interest rate swaps are recorded at fair value. The valuation of our derivatives is determined using widely accepted valuation techniques including discounted cash flow analysis on the expected cash flows of each derivative, which reflects the contractual terms of the derivatives, including the period to maturity, and uses observable market-based inputs, including interest rate curves. We have determined that the majority of the inputs used to value our derivatives fall within Level 2 of the fair value hierarchy; however, the credit valuation adjustments associated with our derivatives utilize Level 3 inputs, such as estimates of current credit spreads, to evaluate the likelihood of default by our Operating Partnership and its counterparties. As of December 31, 2017, we have assessed the significance of the impact of the credit valuation adjustments on the overall valuation of our derivative positions and have determined that the credit valuation adjustment is not significant to the overall valuation of our derivative portfolio. As a result we classify our derivative valuation in Level 2 of the fair value hierarchy. The total principal balance of our revolving credit facility, senior unsecured term loans, and senior unsecured notes was $944.5 million and $694.0 million as of December 31, 2017 and 2016, respectively, with a fair value of $946.6 million and $673.0 million, respectively, based on Level 3 inputs from the fair value hierarchy. Under the discounted cash flow method, the fair values of the revolving credit facility, and the senior unsecured term loans are based on our assumptions of market interest rates and terms available incorporating our credit risk for similar loan maturities.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies.
Commitments and Contingencies	16. Commitments and Contingencies Our properties require periodic investments of capital for general capital improvements and for tenant related capital expenditures. We enter into various construction and equipment contracts with third parties for the development of our properties. At December 31, 2017, we had open commitments related to construction contracts of approximately $113.0 million. Additionally, we have commitments related to telecommunications capacity used to connect data centers located within the same market or geographical area and power usage. At December 31, 2017, we had open commitments related to these contracts of approximately $103.7 million, of which $7.0 million is scheduled to be met during the year ended December 31, 2018. In the ordinary course of business, we are subject to claims and administrative proceedings. Except as described below, we are not presently party to any proceeding, which we believe to be material or which we would expect to have, individually or in the aggregate, a material adverse effect on our business, financial condition, cash flows or results of operations. On February 4, 2014, U.S. Colo, LLC (“U.S. Colo”), a current customer, filed a complaint against us in the United States District Court for the Central District of California. In the complaint, U.S. Colo alleged that it should not have been charged for the use of various interconnection services under the terms of an existing agreement between the parties. On July 23, 2015, after amendments to the complaint, dismissal of several of U.S. Colo’s claims, and a remand of the case to the Superior Court of the State of California, County of Los Angeles (the “Superior Court”), U.S. Colo filed an amended complaint alleging breach of contract and breach of the covenant of good faith and fair dealing. The amended complaint seeks $802,564 in damages for charges paid to us for interconnection services; $70,080,000 in damages for alleged losses of revenue and profits; attorney's fees, interest, and costs of the suit; and declaratory and injunctive relief. On February 4, 2016, the Superior Court granted our motion for summary adjudication, finding that U.S. Colo had contractually waived all claims for lost revenue or profits or other consequential damages. The case was set to start trial on July 24, 2017 (after being rescheduled previously), but upon unopposed application by U.S. Colo, the Superior Court vacated the July 24, 2017, trial date, continued the trial date to April 9, 2018, set a final status conference for March 29, 2018, and reset other procedural deadlines in the action. We intend to vigorously defend this legal proceeding. While it is not feasible to predict or determine the outcome of this legal proceeding, as of December 31, 2017, we estimate that the ultimate resolution of this matter and other disputes could result in a loss that is reasonably possible between zero and $2.5 million in the aggregate, of which $2.3 million has been accrued in accounts payable and accrued expenses in our consolidated balance sheet as of December 31, 2017. On July 9, 2015, a purported class action lawsuit was filed in the Superior Court of the State of California, County of Los Angeles, against us, alleging various employment law violations related to overtime, meal and break periods, minimum wage, timely payment of wages, wage statements, payroll records and business expenses. On March 15, 2016, we filed a responsive pleading generally denying the allegations. On July 27, 2016, the parties agreed upon class-wide settlement terms, subject to court approval. On November 28, 2017, the Superior Court granted final approval of the parties’ settlement, which resolves the matter on a class-wide basis, on behalf of all non-exempt employees in California. The settlement also resolves a related class action lawsuit filed on July 22, 2016, alleging similar claims. As part of the settlement, we agreed to pay $600,000, which was paid on December 12, 2017.

Quarterly Financial Information

Quarterly Financial Information (unaudited)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information (unaudited)
Quarterly Financial Information (unaudited)	17. Quarterly Financial Information (unaudited) The table below reflects the selected quarterly information for the years ended December 31, 2017, and 2016 (in thousands except share data): Three Months Ended December 31, September 30, June 30, March 31, 2017 2017 2017 2017 Revenue $ 125,946 $ 123,068 $ 117,886 $ 114,921 Operating income 33,667 30,799 30,082 30,264 Net income 27,008 24,288 24,135 25,060 Net income attributable to common shares 14,912 15,758 15,643 16,292 Net income per share attributable to common shares - basic $ 0.44 $ 0.47 $ 0.46 $ 0.49 Net income per share attributable to common shares - diluted $ 0.44 $ 0.46 $ 0.46 $ 0.48 Three Months Ended December 31, September 30, June 30, March 31, 2016 2016 2016 2016 Revenue $ 110,508 $ 101,274 $ 96,090 $ 92,480 Operating income 27,933 22,505 22,558 21,621 Net income 23,161 19,319 19,835 19,606 Net income attributable to common shares 14,895 12,180 12,035 11,261 Net income per share attributable to common shares - basic $ 0.45 $ 0.36 $ 0.38 $ 0.37 Net income per share attributable to common shares - diluted $ 0.44 $ 0.36 $ 0.37 $ 0.37

Schedule III Real Estate and Ac

Schedule III Real Estate and Accumulated Depreciation	12 Months Ended
	Dec. 31, 2017
Schedule III Real Estate and Accumulated Depreciation
Schedule III Real Estate and Accumulated Depreciation	CoreSite Realty Corporation Schedule II Real Estate and Accumulated Depreciation December 31, 2017 (in thousands) Gross Amount Carried at Initial Cost Costs Capitalized December 31, 2017 Accumulated Year Building and Subsequent to Building and Depreciation at Acquired Property Name Location Land Improvements Acquisition Land Improvements Total December 31, 2017 or Leased BO1 Somerville, MA $ 6,100 $ 26,748 $ 78,651 $ 5,154 $ 106,345 $ 111,499 $ 51,570 2007 CH1 Chicago, IL 5,493 49,522 62,746 5,493 112,268 117,761 36,057 2010 DC1 Washington, DC — 4,797 4,491 — 9,288 9,288 5,861 2010 DC2 Washington, DC — — 4,405 — 4,405 4,405 — 2017 DE1 Denver, CO — 37 17,676 — 17,713 17,713 2,637 2012 DE2 Denver, CO — 4 1,663 — 1,667 1,667 711 2012 LA1 Los Angeles, CA — 41,099 38,882 — 79,981 79,981 41,224 2010 LA2 Los Angeles, CA 28,467 94,114 143,408 28,467 237,522 265,989 54,603 2010 MI1 Miami, FL 728 9,325 2,315 728 11,640 12,368 3,388 2010 NY1 New York, NY — — 38,008 — 38,008 38,008 22,355 2007 NY2 Secaucus, NJ 4,952 18,408 114,628 2,388 135,600 137,988 21,677 2013 SV1 San Jose, CA 6,863 91,719 41,806 6,863 133,525 140,388 36,235 2010 SV2 Milpitas, CA 5,086 5,046 27,896 5,086 32,942 38,028 19,485 2006 SV3 Santa Clara, CA 4,162 3,482 48,729 3,972 52,401 56,373 34,793 2007 SV4 Santa Clara, CA 4,632 3,716 98,064 4,501 101,911 106,412 38,042 2007 SV5 Santa Clara, CA 2,572 — 23,577 2,544 23,605 26,149 2,545 2007 SV6 Santa Clara, CA 4,741 — 36,826 4,825 36,742 41,567 1,533 2007 SV7 Santa Clara, CA 3,793 — 237,889 3,595 238,087 241,682 16,114 2007 SV8 Santa Clara, CA 12,158 — 1,179 — 13,337 13,337 — 2017 VA1 Reston, VA 6,903 32,939 93,068 6,903 126,007 132,910 62,379 2007 VA2 Reston, VA 5,197 — 132,550 5,539 132,208 137,747 20,870 2007 Reston Campus Expansion Reston, VA 14,200 43,949 31,808 11,200 78,757 89,957 1,062 2016 Total $ 116,047 $ 424,905 $ 1,280,265 $ 97,258 $ 1,723,959 $ 1,821,217 $ 473,141 None of our properties are encumbered. The aggregate cost of the total properties for federal income tax purposes was approximately $1,706.1 million (unaudited) at December 31, 2017. See accompanying report of independent registered public accounting firm. CoreSite Realty Corporation Schedule III Real Estate and Accumulated Depreciation (Continued) December 31, 2017 (in thousands) The following table reconciles the historical cost and accumulated depreciation of our properties for the years ended December 31, 2017, 2016, and 2015: Year Ended December 31, 2017 2016 2015 Property Balance, beginning of year $ 1,643,576 $ 1,295,135 $ 1,146,548 Additions - property acquisitions 12,158 58,149 — Additions - improvements 165,483 290,292 148,587 Balance, end of year $ 1,821,217 $ 1,643,576 $ 1,295,135 Accumulated Depreciation Balance, beginning of year $ 369,303 $ 284,219 $ 215,978 Additions, net of disposals 103,838 85,084 68,241 Balance, end of year $ 473,141 $ 369,303 $ 284,219 See accompanying report of independent registered public accounting firm.

Summary of Significant Accoun26

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Summary of Significant Accounting Policies
Principles of Consolidation and Basis of Presentation	Principles of Consolidation and Basis of Presentation The accompanying consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued guidance codified in Accounting Standards Codification (“ASC”) Topic 606, Revenue Recognition — Revenue from Contracts with Customers , which amends the guidance in former ASC Topic 605, Revenue Recognition . The standard establishes a five-step model framework which recognizes revenue as an entity transfers control of goods or services to the customer and requires enhanced disclosures. This standard does not apply to leases, which will be accounted for under ASC Topic 842, Leases . The revenue standard is effective for interim and annual reporting periods beginning after December 15, 2017. The standard permits the use of either the retrospective or cumulative effect transition method. As ASC Topic 606 does not impact lessor accounting, this standard will not significantly impact our accounting for rental revenue. In addition, there is not a significant impact to our accounting for nonlease components, including power, interconnection, and tenant reimbursement and other revenue. We adopted ASC Topic 606 on January 1, 2018, and will use the cumulative effect transition method. In February 2016, the FASB issued guidance codified in ASC Topic 842, Leases , which amends the guidance in former ASC Topic 840, Leases . We are party to leases as both a lessor and lessee. The main principle of ASC 842 requires lessees to recognize the assets and liabilities that arise from nearly all leases on the consolidated balance sheet. Lessor accounting remains mainly consistent with current guidance, with the majority of changes allowing for better alignment with the new lessee model and ASC Topic 606. The standard is effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. The standard requires a modified retrospective transition approach. We adopted ASC Topic 842 effective January 1, 2018, which would result in a transition date of January 1, 2016. The FASB has issued a Proposed Accounting Standards Update, Leases – Targeted Improvements, which proposes updates to the lease standard and would allow us to apply the new lease standard as of our January 1, 2018, adoption date and remove the requirement for us to restate our prior period financial statements upon adoption. If the proposed accounting standards update is issued in final form, we will apply the transition relief under the new lease standard as of January 1, 2018. We are currently evaluating other impacts of the FASB’s proposed updates to the lease standard, specifically the proposed practical expedient which allows lessors not to separate nonlease components from the related lease components if both the timing and pattern of revenue recognition are the same for the nonlease component(s) and related lease component and the combined single lease component would be classified as an operating lease. As a lessee the classification of our leases will not change, but we will be required to recognize a lease liability and corresponding right-of-use asset on our consolidated balance sheets for all of our operating leases. We have elected the package of practical expedients which allows us to not reassess (1) whether existing contracts contain leases, (2) the lease classification for existing leases, and (3) whether existing initial direct costs meet the new definition. We did not elect the hindsight practical expedient which permits entities to use hindsight in determining the lease term. Using the fixed noncancellable term of all existing data center leases, excluding renewal options, our initial lease liability and right-of-use asset will each be approximately $100 million on our consolidated balance sheet as of January 1, 2018. In accordance with ASC Topic 842, lessor accounting for our leases remains largely unchanged, apart from the narrower definition of initial direct costs that can be capitalized. The new standard defines initial direct costs as only the incremental costs of signing a lease. Internal sales employees’ compensation, payroll-related fringe benefits and certain external legal fees related to the execution of successful lease agreements will not meet the definition of initial direct costs under the new standard and will be accounted for as a sales and marketing expense or general and administrative expense in the consolidated statements of operations upon adoption of ASC Topic 842. For the year ended December 31, 2017, we had $1.2 million and $0.1 million of capitalized salaries and benefits and external legal fees, respectively, which would not have been capitalized if ASC Topic 842 had been in effect. On a prospective basis, we estimate that the sales and marketing expense in our consolidated statement of operations will increase by approximately $1.4 million and general and administrative expense will increase by approximately $0.1 million for the year ending December 31, 2018, as a result of the narrower definition of initial direct costs. In August 2016, the FASB issued guidance codified in ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments. The standard provides guidance on eight specific cash flow classification issues including debt prepayment or debt extinguishment costs, contingent consideration payments made after a business combination, and separately identifiable cash flows and application of the predominance principle. The standard will be effective for the fiscal year beginning January 1, 2018, and subsequent interim periods. We do not expect the provisions of ASU 2016-15 to have a material impact on our consolidated financial statements. In January 2017, the FASB issued guidance codified in ASU 2017-04, Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. ASU 2017-04 simplifies the accounting for goodwill impairment by eliminating the process of measuring the implied value of goodwill, known as step two, from the goodwill impairment test. Instead, if the carrying amount of a reporting unit exceeds its fair value, an impairment loss shall be recognized in an amount equal to that excess, limited to the total amount of goodwill allocated to that reporting unit. The standard will be effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. We do not expect the provisions of ASU 2017-04 to have a material impact on our consolidated financial statements. In August 2017, the FASB issued guidance codified in ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. ASU 2017-12 simplifies hedge accounting by eliminating the requirement to separately measure and report hedge ineffectiveness and presenting all items that affect earnings in the same income statement line item as the hedged item. The standard will be effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted. We do not expect the provisions of ASU 2017-12 to have a material impact on our consolidated financial statements.
Use of Estimates	Use of Estimates The preparation of these consolidated financial statements in conformity with GAAP requires our management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingencies at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. We evaluate our estimates, including those related to assessing the carrying values of our real estate properties, goodwill, accrued liabilities and performance-based equity compensation plans. We base our estimates on historical experience, current market conditions, and various other assumptions that we believe to be reasonable under the circumstances. Actual results may vary from those estimates and those estimates could vary under different assumptions or conditions.
Reclassifications	Reclassifications Certain immaterial amounts included in the consolidated financial statements for 2016 and 2015 have been reclassified to conform to the 2017 financial statement presentation.
Investments in Real Estate	Investments in Real Estate Real estate investments are carried at cost less accumulated depreciation and amortization. The cost of real estate includes the purchase price of property and leasehold improvements. Expenditures for maintenance and repairs are expensed as incurred. Significant renovations and betterments that extend the economic useful lives of assets are capitalized. During land development and construction periods, we capitalize construction costs, legal fees, financing costs, real estate taxes and insurance, rent expense and internal costs of personnel performing development, if such costs are incremental and identifiable to a specific development project. Capitalization of costs begins upon commencement of development efforts and ceases when the project is ready for its intended use and held available for occupancy. Interest is capitalized during the period of development based upon applying the weighted-average borrowing rate to the actual development costs expended. Capitalized interest costs were $3.3 million, $4.1 million and $3.7 million for the years ended December 31, 2017, 2016, and 2015, respectively. Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: Buildings 27 to 40 years Building improvements 1 to 10 years Leasehold improvements The shorter of the lease term or useful life of the asset Depreciation expense was $108.1 million, $89.2 million and $76.1 million for the years ended December 31, 2017, 2016 and 2015, respectively.
Acquisition of Investment in Real Estate	Acquisition of Investment in Real Estate When accounting for business combinations and asset acquisitions, the fair value of the real estate acquired is allocated to the acquired tangible assets, consisting primarily of land, building and building improvements, and identified intangible assets and liabilities, consisting of the value of above-market and below-market leases, value of in-place leases and the value of customer relationships. The fair value of the land and building of an acquired property is determined by valuing the property as if it were vacant, and the “as-if-vacant” fair value is then allocated to land and building based on management’s determination of the fair values of these assets. Management determines the as-if-vacant fair value of a property using methods similar to those used by independent appraisers. Factors considered by management in performing these analyses include an estimate of carrying costs during the expected lease-up periods considering current market conditions and costs to execute similar leases. The fair value of intangibles related to in-place leases includes the value of lease intangibles for above-market and below-market leases, lease origination costs, and customer relationships, determined on a lease-by-lease basis. Above-market and below-market leases are valued based on the present value (using an interest rate which reflects the risks associated with the leases acquired) of the difference between (i) the contractual amounts to be paid pursuant to the in-place leases and (ii) management's estimate of market lease rates for the corresponding in-place leases, measured over a period equal to the remaining noncancelable term of the lease and, for below-market leases, over a time period equal to the initial term plus any below-market fixed rate renewal periods. Lease origination costs include estimates of costs avoided associated with leasing the property, including tenant allowances and improvements and leasing commissions. Customer relationship intangibles relate to the additional revenue opportunities expected to be generated through interconnection services and utility services to be provided to the in-place lease tenants. The capitalized values for above and below-market lease intangibles, lease origination costs, and customer relationships are amortized over the term of the underlying leases or the expected customer relationship. Amortization related to above-market and below-market leases where the Company is the lessor is recorded as either a reduction of or an increase to rental revenue, amortization related to above-market and below-market leases where the Company is the lessee is recorded as either a reduction of or an increase to rent expense. If a lease is terminated prior to its stated expiration, all unamortized amounts relating to that lease are written off. The carrying value of intangible assets is reviewed for impairment in connection with its respective asset group whenever events or changes in circumstances indicate that the asset group may not be recoverable. An impairment loss is recognized if the carrying amount of the asset group is not recoverable and its carrying amount exceeds its estimated fair value. No impairment loss related to these intangible assets was recognized for the years ended December 31, 2017, 2016, or 2015. The excess of the cost of an acquired business over the net of the amounts assigned to assets acquired (including identified intangible assets) and liabilities assumed is recorded as goodwill. As of December 31, 2017, and 2016, we had approximately $40.6 million and $41.2 million of goodwill, respectively. The Company’s goodwill has an indeterminate life and is not amortized, but is tested for impairment on an annual basis, or more frequently if events or changes in circumstances indicate that the asset might be impaired. No impairment loss was recognized for the years ended December 31, 2017, 2016, or 2015. During the year ended December 31, 2017, goodwill was reduced by $0.5 million due to the receipt of cash from an escrow account associated with an entity acquired at the time of the Company’s initial public offering (“IPO”).
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include all non‑restricted cash held in financial institutions and other non‑restricted highly liquid short‑term investments with original maturities at acquisition of three months or less.
Deferred Costs	Deferred Costs Deferred leasing costs include commissions paid to third parties, including brokers, leasing and referral agents, and internal sales commissions paid to employees for successful execution of lease agreements. These commissions and other direct and incremental costs incurred to obtain new customer leases are capitalized and amortized over the term of the related leases using the straight-line method. If a customer lease terminates prior to the expiration of its initial term, any unamortized deferred costs related to the lease are written off to amortization expense. Deferred leasing costs are included within other assets in the consolidated balance sheets and consisted of the following, net of amortization, as of December 31, 2017, and 2016 (in thousands): December 31, December 31, 2017 2016 Internal sales commissions $ 17,402 $ 18,748 Third party commissions 11,802 13,643 External legal counsel 775 730 Total $ 29,979 $ 33,121 Deferred financing costs include costs incurred in connection with obtaining debt and extending existing debt. These financing costs are capitalized and amortized on a straight-line basis, which approximates the effective-interest method, over the term of the loan and the amortization is included as a component of interest expense. Depending on the type of debt instrument, deferred financing costs are reported either in other assets or as a direct deduction from the carrying amount of the related debt liabilities in our consolidated balance sheets.
Recoverability of Long-Lived Assets	Recoverability of Long‑Lived Assets We review our long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Impairment is recognized when estimated expected future cash flows (undiscounted and without interest charges) are less than the carrying amount of the assets. The estimation of expected future net cash flows is inherently uncertain and relies, to a considerable extent, on assumptions regarding current and future economics and market conditions and the availability of capital. If, in future periods, there are changes in the estimates or assumptions incorporated into the impairment review analysis, the changes could result in an adjustment to the carrying amount of the long-lived assets. To the extent that impairment has occurred, the excess of the carrying amount of long-lived assets over its estimated fair value would be recognized as an impairment loss charged to net income. For the years ended December 31, 2017, 2016, and 2015, no impairment of long-lived assets was recognized.
Derivative Instruments and Hedging Activities	Derivative Instruments and Hedging Activities We reflect all derivative instruments at fair value as either assets or liabilities on the consolidated balance sheets. For those derivative instruments that are designated and qualify as hedging instruments, we record the effective portion of the gain or loss on the hedging instruments as a component of accumulated other comprehensive income or loss. Any ineffective portion of a derivative’s change in fair value is immediately recognized within net income. For derivatives that do not meet the criteria for hedge accounting, changes in fair value are immediately recognized within net income. See additional discussion in Note 9 Derivatives and Hedging Activities.
Internal-Use Software	Internal-Use Software We recognize internal-use software development costs based on the development stage of the project and nature of the cost. Internal and external costs incurred during the preliminary project stage are expensed as they are incurred. Internal and external costs incurred to develop internal-use software during the application development stage are capitalized. Internal and external training costs and maintenance costs during the post-implementation-operation stage are expensed as incurred. Completed projects are placed into service and amortized over the estimated useful life of the software. For the years ended December 31, 2017, and 2016, no impairment was recognized related to impairment of internal use software. For the year ended December 31, 2015, we recorded $0.3 million related to impairment of internal use software. Our SaaS arrangements are capitalized when we have a contractual right to take possession of the software at any point during the period without significant penalty and it is feasible to run the software on our own hardware or we are able to contract with an unrelated third party to run the software. Any arrangements that do not meet these criterion are accounted for as a service contract and the costs are expensed as the services are performed.
Revenue Recognition	Revenue Recognition Our customer arrangements contain lease and nonlease elements. Consideration called for by the arrangement is separated at the inception of the arrangement into the lease and nonlease elements based on the relative fair value of each element. For the lease elements, rental revenue is recognized on a straight‑line basis over the customer’s lease term. The excess of rents recognized over amounts contractually due pursuant to the underlying leases is recorded as deferred rent receivable within our consolidated balance sheets. Nonlease elements include power and interconnection services. Power revenue is recognized each month as the power services are delivered to and utilized by our customers. Power services may include non‑recurring customer set‑up charges and installation fees that are initially deferred and recognized over the expected performance period. Interconnection services are contracted on a month-to-month basis and revenue is recognized each month as these services are delivered to and utilized by our customers. Some of our data center leases contain provisions under which our customers reimburse us for a portion of real estate taxes, insurance, common area maintenance, and other recoverable costs. Such customer reimbursements are recognized in the period that the expenses are recognized. A provision for uncollectible accounts is recorded if a receivable balance relating to contractual rent, rent recorded on a straight-line basis, tenant reimbursements or other billed amounts is considered by management to be uncollectible. At December 31, 2017, and 2016, the allowance for doubtful accounts totaled $1.1 million and $0.2 million, respectively.
Share-Based Compensation	Share‑Based Compensation We account for share-based compensation using the fair value method of accounting. The estimated fair value of the stock options granted by us is calculated based on the Black-Scholes option-pricing model. The fair value of restricted share-based and Operating Partnership unit compensation is based on the fair value of our common stock on the date of the grant. The fair value of performance share awards, which have a market condition, is based on a Monte Carlo simulation. The fair value for all share-based compensation is amortized on a straight-line basis over the vesting period. We have elected to account for forfeitures as they occur.
Asset Retirement and Environmental Remediation Obligations	Asset Retirement and Environmental Remediation Obligations We record accruals for estimated asset retirement and environmental remediation obligations. The obligations relate primarily to the removal of asbestos during development of properties as well as the estimated equipment removal costs upon termination of a certain lease where we are the lessee. At December 31, 2017, and 2016, the amount included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.5 million and $1.4 million, respectively.
Income Taxes	Income Taxes We elected to be taxed as a REIT under the Internal Revenue Code of 1986, as amended (the “Code”), commencing with our taxable year ended December 31, 2010. To qualify as a REIT, we are required to distribute at least 90% of our taxable income to our stockholders and meet various other requirements imposed by the Code relating to such matters as operating results, asset holdings, distribution levels and diversity of stock ownership. Provided we qualify for taxation as a REIT, we generally are not subject to corporate level federal income tax on the earnings distributed currently to our stockholders. If we fail to qualify as a REIT in any taxable year, and are unable to avail ourselves of certain savings provisions set forth in the Code, all of our taxable income would be subject to federal income tax at regular corporate rates, including any applicable alternative minimum tax. To maintain REIT status, we must distribute a minimum of 90% of our taxable income. However, it is our policy and intent, subject to change, to distribute 100% of our taxable income and therefore, no provision is required in the accompanying consolidated financial statements for federal income taxes with regards to activities of the REIT and its subsidiary pass‑through entities. The allocable share of taxable income is included in the income tax returns of its stockholders. The Company is subject to the statutory requirements of the locations in which it conducts business. State and local income taxes are accrued as deemed required in the best judgment of management based on analysis and interpretation of respective tax laws. We have elected to treat certain subsidiaries as taxable REIT subsidiaries (“TRS”). Certain activities that we undertake must be conducted by a TRS, such as services for our tenants that could be considered otherwise impermissible for us to perform and holding assets that we cannot hold directly. A TRS is subject to corporate level federal and state income taxes. Deferred income taxes are recognized in certain taxable entities. Deferred income tax generally is a function of the period’s temporary differences (items that are treated differently for tax purposes than for financial reporting purposes), the utilization of tax net operating losses generated in prior years that previously had been recognized as deferred income tax assets and the reversal of any previously recorded deferred income tax liabilities. A valuation allowance for deferred income tax assets is provided if we believe all or some portion of the deferred income tax asset may more likely than not be realized. Any increase or decrease in the valuation allowance resulting from a change in circumstances that causes a change in the estimated realizability of the related deferred income tax asset is included in deferred tax expense. As of December 31, 2017, and 2016, the gross deferred income taxes were not material. We currently have no liabilities for uncertain income tax positions. The earliest tax year for which we are subject to examination is 2014.
Concentration of Credit Risks	Concentration of Credit Risks Our cash and cash equivalents are maintained in various financial institutions, which, at times, may exceed federally insured limits. We have not experienced any losses in such accounts, and management believes that the Company is not exposed to any significant credit risk in this area. We have no off‑balance sheet concentrations of credit risk, such as foreign exchange contracts, option contracts, or foreign currency hedging arrangements.
Segment Information	Segment Information We manage our business as one reportable segment consisting of investments in data centers located in the United States. Although we provide services in several markets, these operations have been aggregated into one reportable segment based on the similar economic characteristics amongst all markets, including the nature of the services provided and the type of customers purchasing these services.

Summary of Significant Accoun27

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Summary of Significant Accounting Policies
Schedule of useful lives of real estate investment assets	Buildings 27 to 40 years Building improvements 1 to 10 years Leasehold improvements The shorter of the lease term or useful life of the asset
Schedule of deferred leasing costs, net of amortization	Deferred leasing costs are included within other assets in the consolidated balance sheets and consisted of the following, net of amortization, as of December 31, 2017, and 2016 (in thousands): December 31, December 31, 2017 2016 Internal sales commissions $ 17,402 $ 18,748 Third party commissions 11,802 13,643 External legal counsel 775 730 Total $ 29,979 $ 33,121

Acquired Intangible Assets an28

Acquired Intangible Assets and Liabilities (Tables)	12 Months Ended
	Dec. 31, 2017
Acquired Intangible Assets and Liabilities
Schedule of above-market and below-market lease intangibles and other intangibles	Above-market and below-market lease intangibles and other intangibles balances at December 31, 2017, and 2016, are as follows (in thousands): December 31, December 31, 2017 2016 Lease contracts above-market value $ 714 $ 2,080 Accumulated amortization (532) (1,698) Lease contracts above-market value, net $ 182 $ 382 Lease contracts below-market value $ 9,102 $ 9,731 Accumulated amortization (5,608) (5,439) Lease contracts below-market value, net $ 3,494 $ 4,292 Other identified intangible assets $ 14,174 $ 20,218 Accumulated amortization (8,053) (10,687) Other identified intangible assets, net $ 6,121 $ 9,531
Schedule of estimated amortization of acquired below-market leases, net of acquired above-market leases	The estimated amortization of acquired below‑market leases, net of acquired above‑market leases for each of the five succeeding fiscal years, which will be a net increase to rental revenue, is as follows (in thousands): Year Ending December 31, 2018 $ 456 2019 236 2020 241 2021 234 2022 208 Thereafter 1,937 Total $ 3,312
The estimated amortization of deferred leasing costs for each of the five succeeding fiscal years	The estimated amortization of all other identified intangible assets for each of the five succeeding fiscal years is as follows (in thousands): Year Ending December 31, 2018 $ 1,931 2019 1,481 2020 897 2021 856 2022 705 Thereafter 251 Total $ 6,121

Investment in Real Estate (Tabl

Investment in Real Estate (Tables)	12 Months Ended
	Dec. 31, 2017
Investment in Real Estate
Summary of properties owned or leased	The following is a summary of the properties owned or leased by market at December 31, 2017 (in thousands): Buildings and Construction in Market Land Improvements Progress Total Cost Boston $ 5,154 $ 106,317 $ 28 $ 111,499 Chicago 5,493 101,798 10,470 117,761 Denver — 19,193 187 19,380 Los Angeles 28,467 273,000 44,503 345,970 Miami 728 11,612 28 12,368 New York 2,388 132,413 41,195 175,996 Northern Virginia (1) 23,642 300,329 50,336 374,307 San Francisco Bay (2) 31,386 616,394 16,156 663,936 Total $ 97,258 $ 1,561,056 $ 162,903 $ 1,821,217 (1) On February 23, 2017, we executed a ten-year lease for 25,000 square feet at a new property, which we refer to as DC2, to further expand our data center presence in Washington D.C., within our Northern Virginia market. DC2 is under construction as of December 31, 2017. (2) On August 29, 2017, we acquired a two-acre land parcel adjacent to our existing Santa Clara campus, with a total real estate cost of $12.2 million. In accordance with the purchase and sale agreement, the seller has committed to pay us $0.3 million over six months to assist them in vacating the property. We will account for the $0.3 million of cash receipts as ancillary operations which will reduce the basis of the land parcel. We plan to build a turn-key data center on the acquired land parcel, which we refer to as SV8, upon the receipt of necessary entitlements.

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Other Assets.
Schedule of other assets	Our other assets consisted of the following, net of amortization and depreciation, if applicable, as of December 31, 2017, and 2016 (in thousands): December 31, December 31, 2017 2016 Deferred rent receivable $ 40,038 $ 36,393 Deferred leasing costs 29,979 33,121 Internal-use software 17,477 14,440 Prepaid expenses 6,770 4,390 Corporate furniture, fixtures and equipment 6,408 5,356 Deferred financing costs - revolving credit facility 957 1,604 Other 1,872 1,068 Total $ 103,501 $ 96,372
Estimated amortization of deferred leasing costs for each of the five succeeding fiscal years	The estimated amortization of deferred leasing costs for each of the five succeeding fiscal years is as follows (in thousands): Year Ending December 31, 2018 $ 13,845 2019 8,194 2020 3,900 2021 1,560 2022 1,019 Thereafter 1,461 Total $ 29,979

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2017
Leases
Schedule of future minimum lease payments to be received under noncancelable operating leases	As a lessor, the Company leases data center space and office and light-industrial space under noncancelable operating lease agreements. The future minimum lease payments to be received under noncancelable operating leases in effect at December 31, 2017, are as follows (in thousands): Year Ending December 31, 2018 $ 237,706 2019 174,763 2020 120,834 2021 92,684 2022 68,259 Thereafter 161,525 Total $ 855,771
Schedule of future minimum lease payments to be paid under noncancelable leases	The future minimum lease payments to be paid under noncancelable real estate leases in effect at December 31, 2017, are as follows (in thousands): Year Ending December 31, 2018 $ 21,264 2019 22,491 2020 22,928 2021 22,676 2022 15,550 Thereafter 11,874 Total $ 116,783

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt
Summary of outstanding indebtedness	A summary of outstanding indebtedness as of December 31, 2017, and 2016, is as follows (in thousands): Maturity December 31, December 31, Interest Rate Date 2017 2016 Revolving credit facility 3.11% and 2.32% at December 31, 2017, and December 31, 2016, respectively June 24, 2019 $ 169,500 $ 194,000 2020 Senior unsecured term loan (1) 3.00% and 2.60% at December 31, 2017, and December 31, 2016, respectively June 24, 2020 150,000 150,000 2021 Senior unsecured term loan 3.06% and 2.27% at December 31, 2017, and December 31, 2016, respectively February 2, 2021 100,000 100,000 2022 Senior unsecured term loan (2) 3.04% and 3.23% at December 31, 2017, and December 31, 2016, respectively April 19, 2022 200,000 100,000 2023 Senior unsecured notes 4.19% at December 31, 2017, and December 31, 2016, respectively June 15, 2023 150,000 150,000 2024 Senior unsecured notes 3.91% at December 31, 2017 April 20, 2024 175,000 — Total principal outstanding ` 944,500 694,000 Unamortized deferred financing costs (4,930) (3,550) Total debt $ 939,570 $ 690,450 (1) Our Operating Partnership has in place a swap agreement with respect to the 2020 Term Loan (as defined below) to swap the variable interest rate associated with $75 million, or 50% of the principal amount, of the 2020 Term Loan to a fixed rate of approximately 2.93% per annum at our current leverage ratio. The interest rate on the remaining $75 million of the 2020 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of December 31, 2017, is 3.00%. See “Derivatives and Hedging Activities” in Note 9 below. (2) Our Operating Partnership has in place a swap agreement with respect to the 2022 Term Loan (as defined below) to swap the variable interest rate associated with $50 million, or 25% of the principal amount, of the 2022 Term Loan to a fixed rate of approximately 2.98% per annum at our current leverage ratio through January 31, 2019. The interest rate on the remaining $150 million of the 2022 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of December 31, 2017, is 3.04%. See “Derivatives and Hedging Activities” in Note 9 below.
Summary of amount of the entity's outstanding debt when such debt currently become due	The following table summarizes the amount of our outstanding debt as of December 31, 2017, based on when such debt currently becomes due (in thousands): Year Ending December 31, 2018 $ — 2019 169,500 2020 150,000 2021 100,000 2022 200,000 Thereafter 325,000 Total principal outstanding 944,500 Unamortized deferred financing costs (4,930) Total debt, net $ 939,570

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity
Schedule of dividends paid per share on Series A Cumulative Preferred Stock and common shares	Year Ended December 31, Record Date 2017 2016 2015 Common Stock: Ordinary income $ 2.13 $ 2.36 $ 1.70 Qualified dividend — — — Capital gains — — — Return of capital 0.91 — — Total dividend (1) $ 3.04 $ 2.36 $ 1.70 Preferred Stock: Ordinary income $ 1.36 $ 1.81 $ 1.81 Qualified dividend — — — Capital gains — — — Total dividend (2) $ 1.36 $ 1.81 $ 1.81 (1) A $0.98 per share or unit distribution was paid in January, all of which will apply to tax year 2018. Of the $2.36 taxable dividend in 2016, $0.80 was paid in January 2017, of which $0.44 will apply to tax year 2017. Of the $1.70 taxable dividend in 2015, $0.53 was paid in January 2016, of which $0.41 will apply to tax year 2016. (2) All outstanding shares and dividends of the 7.25% Series A Cumulative Redeemable Preferred Stock were redeemed at $25.292014 per share at December 12, 2017. Of the $1.81 taxable dividend in 2016, $0.45 was paid in January 2017. Of the $1.81 taxable dividend in 2015, $0.45 was paid in January 2016.

Noncontrolling Interests - Op34

Noncontrolling Interests - Operating Partnership (Tables)	12 Months Ended
	Dec. 31, 2017
Noncontrolling Interests - Operating Partnership
Schedule of ownership interests in our Operating Partnership	December 31, 2017 December 31, 2016 Number of Units Percentage of Total Number of Units Percentage of Total CoreSite Realty Corporation 33,826,264 71.0 % 33,376,568 70.7 % Noncontrolling interests 13,836,336 29.0 13,851,347 29.3 Total 47,662,600 100.0 % 47,227,915 100.0 %

Equity Incentive Award Plan (Ta

Equity Incentive Award Plan (Tables)	12 Months Ended
	Dec. 31, 2017
Equity Incentive Award Plan
Schedule of stock option activity	Number of Weighted Weighted Aggregate Options outstanding, December 31, 2014 758,095 $ 20.94 6.7 years $ 13.7 million Granted — — Exercised (410,322) 20.15 $ 12.0 million Forfeited (7,281) 27.53 Expired (282) 15.23 Options outstanding, December 31, 2015 340,210 $ 21.77 5.8 years $ 11.9 million Granted — — Exercised (74,660) 17.27 $ 3.8 million Forfeited — — Expired — — Options outstanding, December 31, 2016 265,550 $ 22.96 5.0 years $ 15.0 million Granted — — Exercised (197,761) 24.37 $ 13.3 million Forfeited — — Expired — — Options outstanding, December 31, 2017 67,789 $ 19.12 3.4 years $ 6.4 million Exercisable at December 31, 2015 234,943 $ 18.47 5.4 years $ 9.0 million 2016 249,227 $ 22.29 4.9 years $ 14.2 million 2017 67,789 $ 19.12 3.4 years $ 6.4 million
Schedule of unvested shares subject to options	Number of Weighted Unvested balance, December 31, 2014 283,964 $ 7.75 Granted — — Forfeited (7,281) 8.48 Vested (171,416) 6.86 Unvested balance, December 31, 2015 105,267 $ 9.13 Granted — — Forfeited — — Vested (88,944) 8.97 Unvested balance, December 31, 2016 16,323 $ 9.99 Granted — — Forfeited — — Vested (16,323) 9.99 Unvested balance, December 31, 2017 — $ —
Schedule of restricted awards and RSUs	Restricted Stock Awards and Units Weighted Unvested balance, December 31, 2014 394,016 $ 29.10 Granted 187,644 47.41 Forfeited (34,788) 36.06 Vested (169,260) 26.96 Unvested balance, December 31, 2015 377,612 $ 38.51 Granted 182,628 66.69 Forfeited (37,383) 53.87 Vested (199,216) 36.57 Unvested balance, December 31, 2016 323,641 $ 53.84 Granted 139,788 85.79 Forfeited (34,020) 64.00 Vested (139,566) 44.31 Unvested balance, December 31, 2017 289,843 $ 70.71
Schedule of unvested PSAs and their weighted-average fair value at the date of grant	Weighted- Average Fair Performance-Based Restricted Stock Awards Value at Grant Minimum Maximum Target Date Unvested balance, December 31, 2014 — 85,851 57,233 $ 25.89 Granted 11,222 78,553 44,886 61.22 Performance adjustment (1) 9,981 (7,190) (1,465) — Forfeited (1,458) (13,657) (8,572) 43.38 Vested (594) (594) (594) 25.89 Unvested balance, December 31, 2015 19,151 142,963 91,488 $ 42.78 Granted 11,796 82,572 47,184 84.78 Performance adjustment (1) 29,256 — 11,893 — Forfeited (13,474) (55,916) (34,695) 69.12 Vested (1,894) (1,894) (1,894) 42.17 Unvested balance, December 31, 2016 44,835 167,725 113,976 $ 54.24 Granted 7,210 50,472 28,841 105.50 Performance adjustment (1) 62,841 — 23,725 — Forfeited (5,674) (16,836) (11,255) 79.09 Vested (70,518) (70,518) (70,518) 25.89 Unvested balance, December 31, 2017 38,694 130,843 84,769 $ 84.63 (1) Includes the annual adjustment for the number of PSAs earned based on our achievement of relative TSR measured versus the MSCI US REIT Index for the applicable performance periods.
Summary of assumptions used to value performance stock awards	2017 2016 2015 Expected term (in years) 2.81 2.83 2.83 Expected volatility 23.33 % 25.29 % 25.51 % Expected annual dividend (1) — — — Risk-free rate 1.60 % 0.97 % 1.02 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share
Summary of basic and diluted income per share	The following is a summary of basic and diluted net income per share (in thousands, except share and per share amounts): Year Ended December 31, 2017 2016 2015 Net income attributable to common shares $ 62,605 $ 50,371 $ 26,368 Weighted-average common shares outstanding - basic 33,792,759 32,289,414 25,218,500 Effect of potentially dilutive common shares: Stock options 88,399 192,464 302,264 Unvested awards 177,791 250,181 185,804 Weighted-average common shares outstanding - diluted 34,058,949 32,732,059 25,706,568 Net income per share attributable to common shares Basic $ 1.85 $ 1.56 $ 1.05 Diluted $ 1.84 $ 1.54 $ 1.03

Quarterly Financial Informati37

Quarterly Financial Information (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information (unaudited)
Schedule of selected quarterly information	The table below reflects the selected quarterly information for the years ended December 31, 2017, and 2016 (in thousands except share data): Three Months Ended December 31, September 30, June 30, March 31, 2017 2017 2017 2017 Revenue $ 125,946 $ 123,068 $ 117,886 $ 114,921 Operating income 33,667 30,799 30,082 30,264 Net income 27,008 24,288 24,135 25,060 Net income attributable to common shares 14,912 15,758 15,643 16,292 Net income per share attributable to common shares - basic $ 0.44 $ 0.47 $ 0.46 $ 0.49 Net income per share attributable to common shares - diluted $ 0.44 $ 0.46 $ 0.46 $ 0.48 Three Months Ended December 31, September 30, June 30, March 31, 2016 2016 2016 2016 Revenue $ 110,508 $ 101,274 $ 96,090 $ 92,480 Operating income 27,933 22,505 22,558 21,621 Net income 23,161 19,319 19,835 19,606 Net income attributable to common shares 14,895 12,180 12,035 11,261 Net income per share attributable to common shares - basic $ 0.45 $ 0.36 $ 0.38 $ 0.37 Net income per share attributable to common shares - diluted $ 0.44 $ 0.36 $ 0.37 $ 0.37

Organization (Details)

Organization (Details) - Operating partnership	Dec. 31, 2017
Organization
Percentage of ownership interests held by the entity	71.00%
Percentage of ownership interests held by others	29.00%

Summary of Significant Accoun39

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2018
Recent Accounting Pronouncements
Estimated increase in sales and marketing expense		$ 18,176	$ 17,495	$ 15,930
Estimated increase in general and administrative expense		37,548	$ 35,369	$ 34,179
ASU 2016-02
Recent Accounting Pronouncements
Capitalized salaries and benefits		1,200
External legal fees		$ 100
Forecast | ASU 2016-02
Recent Accounting Pronouncements
Right of use asset					$ 100,000
Right of use liability					$ 100,000
Estimated increase in sales and marketing expense	$ 1,400
Estimated increase in general and administrative expense	$ 100

Summary of Significant Accoun40

Summary of Significant Accounting Policies - Real Estate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments in Real Estate
Capitalized interest costs	$ 3.3	$ 4.1	$ 3.7
Depreciation expense	$ 108.1	$ 89.2	$ 76.1
Building | Minimum
Investments in Real Estate
Useful lives	27 years
Building | Maximum
Investments in Real Estate
Useful lives	40 years
Building Improvements | Minimum
Investments in Real Estate
Useful lives	1 year
Building Improvements | Maximum
Investments in Real Estate
Useful lives	10 years

Summary of Significant Accoun41

Summary of Significant Accounting Policies - Deferred Costs & Impairment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of Significant Accounting Policies
Impairment loss related to intangible assets	$ 0	$ 0	$ 0
Goodwill	40,646,000	41,191,000
Impairment loss related to goodwill	0	0	0
Reduction of goodwill	500,000
Deferred Costs
Internal sales commissions	17,402,000	18,748,000
Third party commissions	11,802,000	13,643,000
External legal counsel	775,000	730,000
Total	29,979,000	33,121,000
Recoverability of Long-Lived Assets
Impairment of real estate	$ 0	$ 0	$ 0

Summary of Significant Accoun42

Summary of Significant Accounting Policies - Internal Use Software, RR & IT (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Internal-Use Software
Recognized impairment of internal use software	$ 0	$ 0	$ 300,000
Revenue Recognition
Increase to rental revenue	600,000	500,000	$ 500,000
Allowance for doubtful accounts (in dollars)	1,094,000	209,000
Asset Retirement and Environmental Remediation Obligations
Asset retirement and environmental remediation obligations	$ 1,500,000	$ 1,400,000
Income taxes
Minimum percentage of distribution of taxable income to qualify as REIT	90.00%
Percentage of distribution of taxable income as per company policy	100.00%
Provision for federal income taxes	$ 0
Liabilities for uncertain positions	$ 0

Summary of Significant Accoun43

Summary of Significant Accounting Policies - Concentration of Credit Risks & Segment Information (Details)	12 Months Ended
	Dec. 31, 2017item
Summary of Significant Accounting Policies
Number of off-balance-sheet concentrations of credit risk	0
Segment information
Number of reportable segments	1

Acquisitions (Details)

Acquisitions (Details) $ in Millions	Aug. 29, 2017USD ($)a	Nov. 10, 2016USD ($)aft²	Jan. 31, 2018USD ($)a	Dec. 31, 2017
Acquisitions
Purchase price	$ 12.2	$ 65	$ 4.5
Area of land acquired | a	2	21.75	2
Net rentable space | ft²		611,000
Amount received in accordance with the purchase and sale agreement	$ 0.3
Amount received in accordance with the purchase adn sale agreement, period				6 months

Acquired Intangible Assets an45

Acquired Intangible Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired Intangible Assets and Liabilities
New intangible assets added		$ 7,500
New intangible liabilities added		$ 400
Weighted average life of intangible assets		4 years 3 months 18 days
Weighted average life of intangible liabilities		5 years 8 months 12 days
Lease contracts above-market value	$ 714	$ 2,080
Accumulated amortization	(532)	(1,698)
Lease contracts above-market value, net	182	382
Lease contracts below-market value	9,102	9,731
Accumulated amortization	(5,608)	(5,439)
Lease contracts below-market value, net	3,494	4,292
Other identified intangible assets	14,174	20,218
Accumulated amortization	(8,053)	(10,687)
Other identified intangible assets, net	6,121	9,531
Amortization of above and below Market Leases	600	500	$ 500
Accumulated amortization of below-market lease	5,608	5,439
Estimated amortization of acquired below-market leases, net of acquired above-market leases
2,018	456
2,019	236
2,020	241
2,021	234
2,022	208
Thereafter	1,937
Total	3,312
Estimated amortization of all other identified intangible assets
2,018	1,931
2,019	1,481
2,020	897
2,021	856
2,022	705
Thereafter	251
Total	6,121
Other Intangible Assets [Member]
Acquired Intangible Assets and Liabilities
Amortization of intangible assets	$ 3,400	$ 2,000	$ 2,000

Investment in Real Estate (Deta

Investment in Real Estate (Details) $ in Thousands	Aug. 29, 2017USD ($)a	Feb. 23, 2017ft²	Dec. 31, 2017USD ($)	Dec. 31, 2015USD ($)	Jan. 31, 2018a	Dec. 31, 2016USD ($)	Nov. 10, 2016a
Investment in Real Estate
Land			$ 97,258			$ 100,258
Buildings and Improvements			1,561,056			1,472,580
Construction in progress			162,903			$ 70,738
Total investments in real estate			$ 1,821,217
Gain (loss) on real estate disposal				$ 36
Amount received in accordance with the purchase and sale agreement	$ 300
Amount received in accordance with the purchase adn sale agreement, period			6 months
Area of land | a	2				2		21.75
Boston
Investment in Real Estate
Land			$ 5,154
Buildings and Improvements			106,317
Construction in progress			28
Total investments in real estate			111,499
Chicago
Investment in Real Estate
Land			5,493
Buildings and Improvements			101,798
Construction in progress			10,470
Total investments in real estate			117,761
Denver
Investment in Real Estate
Buildings and Improvements			19,193
Construction in progress			187
Total investments in real estate			19,380
Los Angeles
Investment in Real Estate
Land			28,467
Buildings and Improvements			273,000
Construction in progress			44,503
Total investments in real estate			345,970
Miami
Investment in Real Estate
Land			728
Buildings and Improvements			11,612
Construction in progress			28
Total investments in real estate			12,368
New York
Investment in Real Estate
Land			2,388
Buildings and Improvements			132,413
Construction in progress			41,195
Total investments in real estate			175,996
Northern Virginia
Investment in Real Estate
Land			23,642
Buildings and Improvements			300,329
Construction in progress			50,336
Total investments in real estate			374,307
San Francisco Bay
Investment in Real Estate
Land			31,386
Buildings and Improvements			616,394
Construction in progress			16,156
Total investments in real estate			$ 663,936
SV8 Santa Clara CA
Investment in Real Estate
Real estate value of acquired land and a vacant building	$ 12,200
Area of land | a	2
Cash from seller	$ 300
DC2 Washington DC
Investment in Real Estate
Length of lease		10 years
Net leased space | ft²		25,000

Other Assets (Details)

Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Other Assets.
Deferred rent receivable	$ 40,038	$ 36,393
Deferred leasing costs	29,979	33,121
Internal-use software	17,477	14,440
Prepaid expense	6,770	4,390
Corporate furniture, fixtures and equipment	6,408	5,356
Deferred financing costs - revolving credit facility	957	1,604
Other	1,872	1,068
Total	103,501	$ 96,372
Estimated amortization of deferred leasing costs
2,018	13,845
2,019	8,194
2,020	3,900
2,021	1,560
2,022	1,019
Thereafter	$ 1,461

Leases (Detail)

Leases (Detail) $ in Thousands	Dec. 31, 2017USD ($)
Future minimum lease payments to be received under noncancelable leases
2,018	$ 237,706
2,019	174,763
2,020	120,834
2,021	92,684
2,022	68,259
Thereafter	161,525
Total	855,771
Operating leases
2,018	21,264
2,019	22,491
2,020	22,928
2,021	22,676
2,022	15,550
Thereafter	11,874
Total	$ 116,783

Debt - Summary of Debt (Details

Debt - Summary of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Apr. 09, 2015
Debt
Interest rate as of reporting date (as a percent)			2.93%
Total principal outstanding	$ 944,500	$ 694,000
Unamortized deferred financing costs	(4,930)	(3,550)
Total debt	$ 939,570	$ 690,450
Revolving Credit Facility
Debt
Interest rate as of reporting date (as a percent)	3.11%	2.32%
Total principal outstanding	$ 169,500	$ 194,000
2020 Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.00%	2.60%
Total principal outstanding	$ 150,000	$ 150,000
2021 Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.06%	2.27%
Total principal outstanding	$ 100,000	$ 100,000
2022 Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.04%	3.23%
Total principal outstanding	$ 200,000	$ 100,000
2023 Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	4.19%	4.19%
Total principal outstanding	$ 150,000	$ 150,000
2024 Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.91%
Total principal outstanding	$ 175,000
Operating partnership | 2019 Senior Unsecured Term Loan | Interest Rate Swap
Debt
Percentage of debt hedged		25.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Interest Rate Swap
Debt
Interest rate as of reporting date (as a percent)	2.93%
Amount of debt hedged	$ 75,000
Percentage of debt hedged	50.00%
Operating partnership | 2022 Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	2.98%
Amount of debt not hedged	$ 150,000
Operating partnership | 2022 Senior Unsecured Term Loan | Interest Rate Swap
Debt
Amount of debt hedged	$ 50,000

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility (Details) $ in Thousands	Feb. 02, 2016USD ($)	Jun. 24, 2015USD ($)item	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Debt
Total principal outstanding			$ 944,500	$ 694,000
Revolving Credit Facility
Debt
Total principal outstanding			169,500	194,000
Maximum borrowing capacity			350,000
Borrowed and outstanding			169,500
Letters of credit outstanding			4,900
Amount available for borrowing under the facility			$ 175,600
Leverage ratio (as a percent)			26.70%
Revolving Credit Facility | LIBOR
Debt
Interest rate margin (as a percent)			1.55%
Reference rate			LIBOR
Revolving Credit Facility | LIBOR | Minimum
Debt
Interest rate margin (as a percent)			1.55%
Revolving Credit Facility | LIBOR | Maximum
Debt
Interest rate margin (as a percent)			2.25%
Revolving Credit Facility | Base Rate
Debt
Reference rate			base rate
Revolving Credit Facility | Base Rate | Minimum
Debt
Interest rate margin (as a percent)			0.55%
Revolving Credit Facility | Base Rate | Maximum
Debt
Interest rate margin (as a percent)			1.25%
Credit Agreement
Debt
Number of options available for extensions | item		1
Extension fee (as a percent)		0.10%
Maximum borrowing capacity		$ 600,000
Maximum borrowing capacity including accordion feature		800,000
Leverage ratio, Covenant (as a percent)			60.00%
Secured debt ratio, Covenant (as a percent)			40.00%
Secured debt ratio (as a percent)			0.00%
Fixed charge coverage ratio			6.5
Unhedged variable rate debt ratio, Covenant (as a percent)			30.00%
Unhedged variable rate debt ratio (as a percent)			13.90%
Credit Agreement | Minimum
Debt
Fixed charge coverage ratio, Covenant			1.7
Credit Agreement | Revolving Credit Facility
Debt
Maximum borrowing capacity		350,000
Credit Agreement | Senior Unsecured Term Loan
Debt
Unsecured term loan		$ 150,000
2020 Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 150,000	150,000
Term of loan facility		5 years
Unsecured term loan		$ 150,000
Leverage ratio (as a percent)			26.70%
2020 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			1.50%
Reference rate			LIBOR
2020 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)			1.50%
2020 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)			2.20%
2020 Senior Unsecured Term Loan | Base Rate
Debt
Reference rate			base rate
2020 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)			0.50%
2020 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)			1.20%
2021 Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 100,000	$ 100,000
Term of loan facility	5 years
Unsecured term loan	$ 100,000
Leverage ratio (as a percent)			26.70%
2021 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			1.50%
Reference rate			LIBOR
2021 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)			1.50%
2021 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)			2.20%
2021 Senior Unsecured Term Loan | Base Rate
Debt
Reference rate			base rate
2021 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)			0.50%
2021 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)			1.20%

Debt - Unsecured Debt (Details)

Debt - Unsecured Debt (Details) - USD ($) $ in Thousands	Feb. 02, 2016	Jun. 24, 2015	Jan. 31, 2014	Dec. 31, 2017	Apr. 20, 2017	Apr. 19, 2017	Dec. 31, 2016	Jun. 15, 2016	Apr. 09, 2015
Debt
Total principal outstanding				$ 944,500			$ 694,000
Debt Instrument, Interest Rate at Period End									2.93%
2019 Senior Unsecured Term Loan
Debt
Unsecured term loan			$ 100,000
Term of loan facility			5 years
Accordion feature			$ 100,000
Increase in accordion feature borrowing capacity			$ 200,000
2020 Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000			$ 150,000
Debt Instrument, Interest Rate at Period End				3.00%			2.60%
Unsecured term loan		$ 150,000
Term of loan facility		5 years
Leverage ratio (as a percent)				26.70%
2020 Senior Unsecured Term Loan | LIBOR
Debt
Reference rate				LIBOR
Interest rate margin (as a percent)				1.50%
2020 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.50%
2020 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.20%
2020 Senior Unsecured Term Loan | Base Rate
Debt
Reference rate				base rate
2020 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.50%
2020 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.20%
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Aggregate principal amount				$ 75,000
2021 Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 100,000			$ 100,000
Debt Instrument, Interest Rate at Period End				3.06%			2.27%
Unsecured term loan	$ 100,000
Term of loan facility	5 years
Leverage ratio (as a percent)				26.70%
2021 Senior Unsecured Term Loan | LIBOR
Debt
Reference rate				LIBOR
Interest rate margin (as a percent)				1.50%
2021 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.50%
2021 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.20%
2021 Senior Unsecured Term Loan | Base Rate
Debt
Reference rate				base rate
2021 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.50%
2021 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.20%
2022 Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 200,000			$ 100,000
Debt Instrument, Interest Rate at Period End				3.04%			3.23%
Unsecured term loan						$ 200,000
Increase in accordion feature borrowing capacity						$ 300,000
Leverage ratio (as a percent)				26.70%
2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.50%
2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.10%
2022 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				50.00%
2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.10%
2023 Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000			$ 150,000
Debt Instrument, Interest Rate at Period End				4.19%			4.19%
Aggregate principal amount								$ 150,000
Stated interest rate								4.19%
Percentage of term notes that may be prepaid								100.00%
Percentage of term notes that holders have right to purchase in cash								100.00%
Percentage of term notes that holders may redeem for cash								100.00%
2024 Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 175,000
Debt Instrument, Interest Rate at Period End				3.91%
Aggregate principal amount					$ 175,000
Stated interest rate					3.91%
Percentage of term notes that may be prepaid					100.00%
Percentage of term notes that holders have right to purchase in cash					100.00%
Percentage of term notes that holders may redeem for cash					100.00%
Operating partnership | 2022 Senior Unsecured Term Loan
Debt
Debt Instrument, Interest Rate at Period End				2.98%

Debt - Maturities (Details)

Debt - Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt maturities
2,019	$ 169,500
2,020	150,000
2,021	100,000
2,022	200,000
Thereafter	325,000
Total principal outstanding	944,500	$ 694,000
Unamortized deferred financing costs	(4,930)	(3,550)
Total debt	$ 939,570	$ 690,450

Derivatives and Hedging Activ53

Derivatives and Hedging Activities (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Apr. 21, 2017	Apr. 09, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Apr. 20, 2018	Feb. 03, 2014
Derivative instruments and hedging activities disclosures
Amount of gain (loss) recorded in accumulated other comprehensive income				$ 580	$ (1,112)	$ (2,426)
Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Earnings related to derivatives due to hedge ineffectiveness					0	0	$ 0
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Amount borrowed under term loan or revolving credit facility			$ 75,000					$ 50,000	$ 100,000
Term of loan facility			5 years
Derivative, Fixed Interest Rate			2.93%					2.98%
Interest rate swap terminated		$ 50,000
Amount of gain (loss) recorded in accumulated other comprehensive income				600	1,100	2,400
Amount reclassified to interest expense				600	1,700	$ 1,900
Amount recognized in unearned revenue, prepaid rent and other liabilities				0	400
Derivative asset recognized in other assets				$ 1,100	$ 300
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging | Forecast
Derivative instruments and hedging activities disclosures
Amount estimated to be reclassified as an increase to interest expense	$ 300
LIBOR | Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Reference rate			one-month LIBOR

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	Dec. 12, 2017	Dec. 12, 2012	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity
Stock authorized to be issued (in shares)			120,000,000
Common stock, shares authorized			100,000,000	100,000,000
Preferred stock authorized to be issued (in shares)			20,000,000	20,000,000
Common Stock, shares issued			34,240,815	33,896,771
Common Stock, shares outstanding			34,240,815	33,896,771
Number of Operating Partnership Units (in shares)			47,662,600	47,227,915
Series A Cumulative Preferred Stock, shares issued			0	4,600,000
Redemption of preferred stock			$ 115,000,000
Proceeds from exercise of stock options			$ 4,820,000	$ 1,290,000	$ 1,393,000
Series A Cumulative Preferred Stock, dividend rate (as a percent)			7.25%	7.25%
Series A Cumulative Preferred Stock, liquidation preference (in dollars per share)			$ 25	$ 25
2010 Plan
Stockholders' Equity
Amount of shares of common stock issued under the plan			$ 329,033	$ 239,068	393,337
Proceeds from exercise of stock options			$ 4,800,000	$ 1,300,000
Payments made for exercise of stock options					$ (2,900,000)
Operating partnership
Stockholders' Equity
Common Operating Partnership units redeemed for common stock (in shares)			15,011	7,000
Redemption of noncontrolling interests (in shares)				3,000,000	8,500,000
Series A Cumulative Redeemable Preferred Stock
Stockholders' Equity
Accrued and unpaid dividends (in dollars per share)	$ 0.292014
Series A Cumulative Preferred Stock, shares issued		4,600,000
Net Proceeds from issuance of preferred stock		$ 110,600,000
Cumulative dividends each year on preferred stock (in dollars per share)		$ 1.8125
Redemption price of preferred stock (in dollars per share)	$ 25
Preferred stock redeemed (in shares)	4,600,000
Total price per share	$ 25.292014
Redemption of preferred stock	$ 116,300,000
Series A Cumulative Preferred Stock, dividend rate (as a percent)		7.25%
Series A Cumulative Preferred Stock, liquidation preference (in dollars per share)		$ 25

Stockholders' Equity - Dividend

Stockholders' Equity - Dividends (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016	Dec. 31, 2015
Subsequent Event
Dividends
Ordinary dividend paid (in dollars per share)			$ 0.98
Minimum
Dividends
Common stock distributions as percentage of REIT taxable income	90.00%
Common stock distributions as percentage of net income (after tax), from foreclosure property	90.00%
Common Stock
Dividends
Ordinary income (in dollars per share)	$ 2.13	$ 2.36				$ 1.70
Return of capital	0.91
Total dividend (in dollars per share)	3.04	2.36				1.70
Dividend declared taxable	0.44	0.41
Ordinary dividend paid (in dollars per share)				$ 0.80	$ 0.53
Preferred Stock
Dividends
Ordinary income (in dollars per share)	1.36	1.81				1.81
Total dividend (in dollars per share)	$ 1.36	$ 1.81				$ 1.81
Ordinary dividend paid (in dollars per share)				$ 0.45	$ 0.45

Noncontrolling Interests - Op56

Noncontrolling Interests - Operating Partnership (Details) $ / shares in Units, $ in Thousands	Dec. 29, 2017$ / shares	Sep. 29, 2017$ / shares	Jun. 30, 2017$ / shares	Mar. 31, 2017$ / shares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)shares
Noncontrolling Interests - Operating Partnership
Conversion ratio for common operating partnership units					1
Series A Cumulative Preferred Stock, dividend rate (as a percent)					7.25%	7.25%
Number of Operating Partnership Units (in shares)					47,662,600	47,227,915
Percentage of Total					100.00%	100.00%
Shares of common stock issued related to employee compensation arrangements					449,696
Coresite Realty Corporation
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)					33,826,264	33,376,568
Percentage of Total					71.00%	70.70%
Noncontrolling Interests
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)					13,836,336	13,851,347
Percentage of Total					29.00%	29.30%
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $					$ 167	$ 37,374	$ 110,180
Operating partnership
Noncontrolling Interests - Operating Partnership
Percentage of Total					71.00%
Percentage of Total					29.00%
Quarterly distributions will receive by holders of operating partnership units (in dollars per unit) | $ / shares	$ 0.98	$ 0.90	$ 0.90	$ 0.80
Common Operating Partnership units redeemed for common stock (in shares)					15,011	7,000
Redemption of noncontrolling interests (in shares)						3,000,000	8,500,000
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $					$ 200	$ 37,400	$ 110,200
Operating partnership | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)						3,000,000	8,500,000
Operating partnership | Noncontrolling Interests
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					15,011	3,007,000	8,500,000
Redemption of noncontrolling interests (in shares)					15,011	3,007,000	8,500,000
Redemption value of the noncontrolling interests | $					$ 1,600,000
Closing price of the Company's stock (in dollars per share) | $ / shares					$ 113.90

Equity Incentive Award Plan (De

Equity Incentive Award Plan (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity Incentive Plan
Shares of common stock reserved for issuance	6,000,000
Shares of common stock that were available for issuance	3,048,508
Stock Options
Number of Shares Subject to Option
Options outstanding at the beginning of the period (in shares)	265,550	340,210	758,095
Exercised (in shares)	(197,761)	(74,660)	(410,322)
Forfeited (in shares)			(7,281)
Expired (in shares)			(282)
Options outstanding at the end of the period (in shares)	67,789	265,550	340,210	758,095
Exercisable at the end of the period (in shares)	67,789	249,227	234,943
Number of Shares Subject to Option
Unvested balance at the beginning of the period (in shares)	16,323	105,267	283,964
Forfeited (in shares)			(7,281)
Vested (in shares)	(16,323)	(88,944)	(171,416)
Unvested balance at the end of the period (in shares)		16,323	105,267	283,964
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 9.99	$ 9.13	$ 7.75
Forfeited (in dollars per share)			8.48
Vested (in dollars per share)	$ 9.99	8.97	6.86
Unvested balance at the end of the period (in dollars per share)		$ 9.99	$ 9.13	$ 7.75
Weighted Average Remaining Contractual Term
Options outstanding at the end of the period	3 years 4 months 24 days	5 years	5 years 9 months 18 days	6 years 8 months 12 days
Exercisable at the end of the period	3 years 4 months 24 days	4 years 10 months 24 days	5 years 4 months 24 days
Aggregate Intrinsic Value
Exercised	$ 13.3	$ 3.8	$ 12
Outstanding at the end of the period		15	11.9	$ 13.7
Exercisable at the end of the period	$ 6.4	$ 14.2	$ 9
Stock Options | Weighted Average
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 22.96	$ 21.77	$ 20.94
Exercised (in dollars per share)	24.37	17.27	20.15
Forfeited (in dollars per share)			27.53
Expired (in dollars per share)			15.23
Options outstanding at the end of the period (in dollars per share)	19.12	22.96	21.77	$ 20.94
Exercisable at the end of the period (in dollars per share)	$ 19.12	$ 22.29	$ 18.47
Restricted Awards
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	323,641	377,612	394,016
Granted (in shares)	139,788	182,628	187,644
Forfeited (in shares)	(34,020)	(37,383)	(34,788)
Vested (in shares)	(139,566)	(199,216)	(169,260)
Unvested balance at the end of the period (in shares)	289,843	323,641	377,612	394,016
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 53.84	$ 38.51	$ 29.10
Granted (in dollars per share)	85.79	66.69	47.41
Forfeited (in dollars per share)	64	53.87	36.06
Vested (in dollars per share)	44.31	36.57	26.96
Unvested balance at the end of the period (in dollars per share)	$ 70.71	53.84	38.51	$ 29.10
Compensation cost not yet recognized
Unearned compensation	$ 14.4
Restricted Awards | Weighted Average
Compensation cost not yet recognized
Vesting period	2 years 4 months 24 days
PSAs
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 54.24	42.78	25.89
Granted (in dollars per share)	105.50	84.78	61.22
Forfeited (in dollars per share)	79.09	69.12	43.38
Vested (in dollars per share)	25.89	42.17	25.89
Unvested balance at the end of the period (in dollars per share)	$ 84.63	$ 54.24	$ 42.78	$ 25.89
Compensation cost not yet recognized
Unearned compensation	$ 3.3
Assumptions used to value the stock options granted
Expected term	2 years 9 months 22 days	2 years 9 months 29 days	2 years 9 months 29 days
Expected volatility (as a percent)	23.33%	25.29%	25.51%
Risk-free rate (as a percent)	1.60%	0.97%	1.02%
PSAs | 2010 Plan
Assumptions used to value the stock options granted
Performance period	3 years
PSAs | Weighted Average
Compensation cost not yet recognized
Vesting period	1 year 9 months 18 days
PSAs | Minimum
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	44,835	19,151
Granted (in shares)	7,210	11,796	11,222
Performance adjustment	62,841	29,256	9,981
Forfeited (in shares)	(5,674)	(13,474)	(1,458)
Vested (in shares)	(70,518)	(1,894)	(594)
Unvested balance at the end of the period (in shares)	38,694	44,835	19,151
PSAs | Minimum | 2010 Plan
Compensation cost not yet recognized
Percentage of target number of shares for PSAs granted in years 2015, 2016 and 2017	25.00%
PSAs | Maximum
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	167,725	142,963	85,851
Granted (in shares)	50,472	82,572	78,553
Performance adjustment			(7,190)
Forfeited (in shares)	(16,836)	(55,916)	(13,657)
Vested (in shares)	(70,518)	(1,894)	(594)
Unvested balance at the end of the period (in shares)	130,843	167,725	142,963	85,851
PSAs | Maximum | 2010 Plan
Compensation cost not yet recognized
Percentage of target number of shares for PSAs granted in years 2015, 2016 and 2017	175.00%
PSAs | Target
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	113,976	91,488	57,233
Granted (in shares)	28,841	47,184	44,886
Performance adjustment	23,725	11,893	(1,465)
Forfeited (in shares)	(11,255)	(34,695)	(8,572)
Vested (in shares)	(70,518)	(1,894)	(594)
Unvested balance at the end of the period (in shares)	84,769	113,976	91,488	57,233

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share
Net income attributable to common shares	$ 14,912	$ 15,758	$ 15,643	$ 16,292	$ 14,895	$ 12,180	$ 12,035	$ 11,261	$ 62,605	$ 50,371	$ 26,368
Weighted-average common shares outstanding - basic									33,792,759	32,289,414	25,218,500
Effect of potentially dilutive common shares:
Stock options									88,399	192,464	302,264
Unvested awards									177,791	250,181	185,804
Weighted-average common shares outstanding - diluted									34,058,949	32,732,059	25,706,568
Net income per share attributable to common shares
Basic (in dollars per share)	$ 0.44	$ 0.47	$ 0.46	$ 0.49	$ 0.45	$ 0.36	$ 0.38	$ 0.37	$ 1.85	$ 1.56	$ 1.05
Diluted (in dollars per share)	$ 0.44	$ 0.46	$ 0.46	$ 0.48	$ 0.44	$ 0.36	$ 0.37	$ 0.37	$ 1.84	$ 1.54	$ 1.03
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share									24,961	29,618	117,048

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employee Benefit Plan
Months of service required in order to receive employer contribution	6 months
Safe harbor contribution as a percentage of employee gross pay (as a percent)	3.00%
Company contributions under certain 401(k) savings plans	$ 1.1	$ 0.9	$ 0.8

Estimated Fair Value of Finan60

Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Estimated Fair Value of Financial Instruments
Total principal outstanding	$ 944,500	$ 694,000
Significant Unobservable Inputs (Level 3)
Estimated Fair Value of Financial Instruments
Fair value of debt	$ 946,600	$ 673,000

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($)	Dec. 12, 2017	Jul. 23, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commitments and Contingencies
Settllement amount	$ 600,000
Maximum
Commitments and Contingencies
Accrued Litigation Liability					$ 2,500,000
Telecommunications Capacity
Commitments and Contingencies
Open commitments				$ 103,700,000
Telecommunications Capacity | Forecast
Commitments and Contingencies
Open commitments met			$ 7,000,000
Construction Contracts
Commitments and Contingencies
Open commitments				113,000,000
Damages Related To Services Rendered
Commitments and Contingencies
Loss Contingency, Damages Sought		$ 802,564
Damages For Loss Of Revenue And Other Costs
Commitments and Contingencies
Alleged losses		$ 70,080,000
Pending Litigation | Minimum
Commitments and Contingencies
Accrued Litigation Liability				0
Pending Litigation | Maximum
Commitments and Contingencies
Accrued Litigation Liability				2,500,000
Pending Litigation | Accounts Payable and Accrued Expenses
Commitments and Contingencies
Accrued Litigation Liability				$ 2,300,000

Quarterly Financial Informati62

Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information (unaudited)
Revenue	$ 125,946	$ 123,068	$ 117,886	$ 114,921	$ 110,508	$ 101,274	$ 96,090	$ 92,480	$ 481,821	$ 400,352	$ 333,292
Operating income	33,667	30,799	30,082	30,264	27,933	22,505	22,558	21,621	124,812	94,617	64,084
Net income	27,008	24,288	24,135	25,060	23,161	19,319	19,835	19,606	100,491	81,921	56,859
Net income attributable to common shares	$ 14,912	$ 15,758	$ 15,643	$ 16,292	$ 14,895	$ 12,180	$ 12,035	$ 11,261	$ 62,605	$ 50,371	$ 26,368
Net income per share attributable to common shares - basic (in dollars per share)	$ 0.44	$ 0.47	$ 0.46	$ 0.49	$ 0.45	$ 0.36	$ 0.38	$ 0.37	$ 1.85	$ 1.56	$ 1.05
Net income per share attributable to common shares - diluted (in dollars per share)	$ 0.44	$ 0.46	$ 0.46	$ 0.48	$ 0.44	$ 0.36	$ 0.37	$ 0.37	$ 1.84	$ 1.54	$ 1.03

Schedule III Real Estate and 63

Schedule III Real Estate and Accumulated Depreciation - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Initial Cost
Land	$ 116,047
Building and Improvements	424,905
Costs Capitalized Subsequent to Acquisition	1,280,265
Gross Amount Carried As of Year End
Land	97,258
Building and Improvements	1,723,959
Total	1,821,217	$ 1,643,576	$ 1,295,135	$ 1,146,548
Accumulated Depreciation	473,141	$ 369,303	$ 284,219	$ 215,978
Aggregate cost of the total properties for federal income tax purposes	1,706,100
BO1 Somerville MA
Initial Cost
Land	6,100
Building and Improvements	26,748
Costs Capitalized Subsequent to Acquisition	78,651
Gross Amount Carried As of Year End
Land	5,154
Building and Improvements	106,345
Total	111,499
Accumulated Depreciation	51,570
CH1 Chicago IL
Initial Cost
Land	5,493
Building and Improvements	49,522
Costs Capitalized Subsequent to Acquisition	62,746
Gross Amount Carried As of Year End
Land	5,493
Building and Improvements	112,268
Total	117,761
Accumulated Depreciation	36,057
DC1 Washington DC
Initial Cost
Building and Improvements	4,797
Costs Capitalized Subsequent to Acquisition	4,491
Gross Amount Carried As of Year End
Building and Improvements	9,288
Total	9,288
Accumulated Depreciation	5,861
DC2 Washington DC
Initial Cost
Costs Capitalized Subsequent to Acquisition	4,405
Gross Amount Carried As of Year End
Building and Improvements	4,405
Total	4,405
DE1 Denver CO
Initial Cost
Building and Improvements	37
Costs Capitalized Subsequent to Acquisition	17,676
Gross Amount Carried As of Year End
Building and Improvements	17,713
Total	17,713
Accumulated Depreciation	2,637
DE2 Denver CO
Initial Cost
Building and Improvements	4
Costs Capitalized Subsequent to Acquisition	1,663
Gross Amount Carried As of Year End
Building and Improvements	1,667
Total	1,667
Accumulated Depreciation	711
LA1 Los Angeles CA
Initial Cost
Building and Improvements	41,099
Costs Capitalized Subsequent to Acquisition	38,882
Gross Amount Carried As of Year End
Building and Improvements	79,981
Total	79,981
Accumulated Depreciation	41,224
LA2 Los Angeles CA
Initial Cost
Land	28,467
Building and Improvements	94,114
Costs Capitalized Subsequent to Acquisition	143,408
Gross Amount Carried As of Year End
Land	28,467
Building and Improvements	237,522
Total	265,989
Accumulated Depreciation	54,603
MI1 Miami FL
Initial Cost
Land	728
Building and Improvements	9,325
Costs Capitalized Subsequent to Acquisition	2,315
Gross Amount Carried As of Year End
Land	728
Building and Improvements	11,640
Total	12,368
Accumulated Depreciation	3,388
NY1 New York NY
Initial Cost
Costs Capitalized Subsequent to Acquisition	38,008
Gross Amount Carried As of Year End
Building and Improvements	38,008
Total	38,008
Accumulated Depreciation	22,355
NY2 Secaucus NJ
Initial Cost
Land	4,952
Building and Improvements	18,408
Costs Capitalized Subsequent to Acquisition	114,628
Gross Amount Carried As of Year End
Land	2,388
Building and Improvements	135,600
Total	137,988
Accumulated Depreciation	21,677
SV1 San Jose CA
Initial Cost
Land	6,863
Building and Improvements	91,719
Costs Capitalized Subsequent to Acquisition	41,806
Gross Amount Carried As of Year End
Land	6,863
Building and Improvements	133,525
Total	140,388
Accumulated Depreciation	36,235
SV2 Milpitas CA
Initial Cost
Land	5,086
Building and Improvements	5,046
Costs Capitalized Subsequent to Acquisition	27,896
Gross Amount Carried As of Year End
Land	5,086
Building and Improvements	32,942
Total	38,028
Accumulated Depreciation	19,485
SV3 Santa Clara CA
Initial Cost
Land	4,162
Building and Improvements	3,482
Costs Capitalized Subsequent to Acquisition	48,729
Gross Amount Carried As of Year End
Land	3,972
Building and Improvements	52,401
Total	56,373
Accumulated Depreciation	34,793
SV4 Santa Clara CA
Initial Cost
Land	4,632
Building and Improvements	3,716
Costs Capitalized Subsequent to Acquisition	98,064
Gross Amount Carried As of Year End
Land	4,501
Building and Improvements	101,911
Total	106,412
Accumulated Depreciation	38,042
SV5 Santa Clara CA
Initial Cost
Land	2,572
Costs Capitalized Subsequent to Acquisition	23,577
Gross Amount Carried As of Year End
Land	2,544
Building and Improvements	23,605
Total	26,149
Accumulated Depreciation	2,545
SV6 Santa Clara CA
Initial Cost
Land	4,741
Costs Capitalized Subsequent to Acquisition	36,826
Gross Amount Carried As of Year End
Land	4,825
Building and Improvements	36,742
Total	41,567
Accumulated Depreciation	1,533
SV7 Santa Clara CA
Initial Cost
Land	3,793
Costs Capitalized Subsequent to Acquisition	237,889
Gross Amount Carried As of Year End
Land	3,595
Building and Improvements	238,087
Total	241,682
Accumulated Depreciation	16,114
SV8 Santa Clara CA
Initial Cost
Land	12,158
Costs Capitalized Subsequent to Acquisition	1,179
Gross Amount Carried As of Year End
Building and Improvements	13,337
Total	13,337
VA1 Reston VA
Initial Cost
Land	6,903
Building and Improvements	32,939
Costs Capitalized Subsequent to Acquisition	93,068
Gross Amount Carried As of Year End
Land	6,903
Building and Improvements	126,007
Total	132,910
Accumulated Depreciation	62,379
VA2 Reston VA
Initial Cost
Land	5,197
Costs Capitalized Subsequent to Acquisition	132,550
Gross Amount Carried As of Year End
Land	5,539
Building and Improvements	132,208
Total	137,747
Accumulated Depreciation	20,870
Reston Campus Expansion
Initial Cost
Land	14,200
Building and Improvements	43,949
Costs Capitalized Subsequent to Acquisition	31,808
Gross Amount Carried As of Year End
Land	11,200
Building and Improvements	78,757
Total	89,957
Accumulated Depreciation	$ 1,062

Schedule III Real Estate and 64

Schedule III Real Estate and Accumulated Depreciation - Rollforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property
Balance, beginning of year	$ 1,643,576	$ 1,295,135	$ 1,146,548
Additions - property acquisitions	12,158	58,149
Additions - improvements	165,483	290,292	148,587
Balance, end of year	1,821,217	1,643,576	1,295,135
Accumulated Depreciation
Balance, beginning of year	369,303	284,219	215,978
Additions, net of disposals	103,838	85,084	68,241
Balance, end of year	$ 473,141	$ 369,303	$ 284,219