CoreSite Realty 10-K 8 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 06, 2019	Jun. 29, 2018
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, 2018
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 Small Business	false
Entity Emerging Growth Company	false
Entity Public Float			$ 2,807.5
Entity Common Stock, Shares Outstanding		36,708,691
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments in real estate:
Land	$ 86,955	$ 97,258
Buildings and improvements	1,730,329	1,561,056
Total investments in real estate	1,817,284	1,658,314
Less: Accumulated depreciation and amortization	(590,784)	(473,141)
Net investment in operating properties	1,226,500	1,185,173
Construction in progress	265,921	162,903
Net investments in real estate	1,492,421	1,348,076
Operating lease right-of-use assets	190,304	92,984
Cash and cash equivalents	2,599	5,247
Accounts and other receivables, net of allowance for doubtful accounts of $417 and $1,094 as of December 31, 2018, and December 31, 2017, respectively	18,464	28,875
Lease intangibles, net of accumulated amortization of $7,756 and $8,585 as of December 31, 2018, and December 31, 2017, respectively	6,943	6,314
Goodwill	40,646	40,646
Other assets, net	102,290	103,501
Total assets	1,853,667	1,625,643
Liabilities:
Debt, net of unamortized deferred financing costs of $5,677 and $4,930 as of December 31, 2018, and December 31, 2017, respectively	1,130,823	939,570
Operating lease liabilities	202,699	102,912
Accounts payable and accrued expenses	89,315	77,170
Accrued dividends and distributions	55,679	48,976
Acquired below-market lease contracts, net of accumulated amortization of $3,840 and $5,608 as of December 31, 2018, and December 31, 2017, respectively	2,846	3,504
Unearned revenue, prepaid rent and other liabilities	37,672	34,867
Total liabilities	1,519,034	1,206,999
Stockholders' equity:
Common Stock, par value $0.01, 100,000,000 shares authorized and 36,708,691 and 34,240,815 shares issued and outstanding at December 31, 2018, and December 31, 2017, respectively	363	338
Additional paid-in capital	491,314	457,495
Accumulated other comprehensive income (loss)	(2,193)	753
Distributions in excess of net income	(246,929)	(177,566)
Total stockholders' equity	242,555	281,020
Noncontrolling interests	92,078	137,624
Total equity	334,633	418,644
Total liabilities and equity	$ 1,853,667	$ 1,625,643

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
CONDENSED CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts (in dollars)	$ 417	$ 1,094
Accumulated amortization, lease intangibles (in dollars)	7,756	8,585
Unamortized Debt Issuance Expense	5,677	4,930
Accumulated amortization, acquired below-market lease contracts (in dollars)	$ 3,840	$ 5,608
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	36,708,691	34,240,815
Common Stock, shares outstanding	36,708,691	34,240,815

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Data center revenue:
Rental revenue	$ 474,683	$ 419,528	$ 347,275
Total operating revenues	544,392	481,821	400,352
Operating expenses:
Property operating and maintenance	152,357	132,820	107,212
Real estate taxes and insurance	19,239	14,913	14,250
Depreciation and amortization	141,633	129,251	108,652
Sales and marketing	21,023	18,176	17,495
General and administrative	40,090	37,548	35,369
Rent	27,696	24,125	22,631
Transaction costs	75	176	126
Total operating expenses	402,113	357,009	305,735
Operating income	142,279	124,812	94,617
Interest expense	(35,526)	(24,147)	(12,577)
Income before income taxes	106,753	100,665	82,040
Income tax (expense) benefit	10	(174)	(119)
Net income	106,763	100,491	81,921
Net income attributable to noncontrolling interests	28,841	25,636	23,212
Net income attributable to CoreSite Realty Corporation	77,922	74,855	58,709
Preferred stock dividends		(7,924)	(8,338)
Original issuance costs associated with redeemed preferred stock		(4,326)
Net income attributable to common shares	$ 77,922	$ 62,605	$ 50,371
Net income per share attributable to common shares:
Basic (in dollars per share)	$ 2.23	$ 1.85	$ 1.56
Diluted (in dollars per share)	$ 2.22	$ 1.84	$ 1.54
Weighted average common shares outstanding
Basic (in shares)	34,957,244	33,792,759	32,289,414
Diluted (in shares)	35,137,495	34,058,949	32,732,059
Rental Power And Related Revenue
Data center revenue:
Rental revenue	$ 463,086	$ 407,680	$ 338,687
Interconnection Revenue
Data center revenue:
Revenue	69,709	62,293	53,077
Office Light Industrial And Other Revenue
Data center revenue:
Rental revenue	$ 11,597	$ 11,848	$ 8,588

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 106,763	$ 100,491	$ 81,921
Other comprehensive income (loss):
Unrealized gain (loss) on derivative contracts	(3,676)	580	(1,112)
Reclassification of other comprehensive income (loss) to interest expense	(326)	623	1,740
Comprehensive income	102,761	101,694	82,549
Comprehensive income attributable to noncontrolling interests	27,912	25,985	23,242
Comprehensive income attributable to CoreSite Realty Corporation	$ 74,849	$ 75,709	$ 59,307

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, 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		$ 1	1,289			1,290		1,290
Exercise of stock options (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				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		$ 2	4,818			4,820		4,820
Exercise of stock options (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				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
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests		$ 23	20,852	127		21,002	(21,002)
Redemption of noncontrolling interests (in shares)		2,261,068
Issuance of stock awards, net of forfeitures (in shares)		188,152
Exercise of stock options			310			310		310
Exercise of stock options (in shares)		18,656
Share-based compensation		$ 2	12,657			12,659		12,659
Dividends and distributions					(147,285)	(147,285)	(52,456)	(199,741)
Net income					77,922	77,922	28,841	106,763
Other comprehensive income				(3,073)		(3,073)	(929)	(4,002)
Balance at Dec. 31, 2018		$ 363	$ 491,314	$ (2,193)	$ (246,929)	$ 242,555	$ 92,078	$ 334,633
Balance (in shares) at Dec. 31, 2018		36,708,691

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 106,763	$ 100,491	$ 81,921
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	141,633	129,251	108,652
Amortization of above/below market leases	(580)	(598)	(541)
Amortization of deferred financing costs	2,370	1,676	1,333
Share-based compensation	12,038	8,946	8,892
Bad debt expense	(229)	1,338	398
Changes in operating assets and liabilities:
Accounts receivable	10,923	(5,087)	(13,199)
Deferred rent receivable	(3,124)	(3,645)	(7,083)
Deferred leasing costs	(11,672)	(13,315)	(14,395)
Other assets	(4,079)	(10,924)	(8,819)
Accounts payable and accrued expenses	3,502	1,224	4,910
Unearned revenue, prepaid rent and other liabilities	(1,275)	(2,235)	8,696
Operating leases	2,491	2,234	(240)
Net cash provided by operating activities	258,761	209,356	170,525
CASH FLOWS FROM INVESTING ACTIVITIES
Tenant improvements	(5,247)	(7,850)	(5,878)
Real estate improvements	(239,715)	(153,956)	(293,359)
Business combinations and asset acquisitions	(10,681)	(12,158)	(64,965)
Net cash used in investing activities	(255,643)	(173,964)	(364,202)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercise of stock options	310	4,820	1,290
Proceeds from revolving credit facility	253,036	274,000	299,500
Payments on revolving credit facility	(211,036)	(298,500)	(247,750)
Proceeds from unsecured debt	150,000	275,000	250,000
Payments of loan fees and costs	(5,038)	(2,410)	(2,981)
Redemption of preferred stock		(115,000)
Dividends and distributions	(193,038)	(172,484)	(108,807)
Net cash provided by (used in) financing activities	(5,766)	(34,574)	191,252
Net change in cash and cash equivalents	(2,648)	818	(2,425)
Cash and cash equivalents, beginning of year	5,247	4,429	6,854
Cash and cash equivalents, end of year	2,599	5,247	4,429
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest, net of capitalized amounts	32,579	22,252	11,528
Cash paid for operating lease liabilities	21,994	20,764	20,184
NON-CASH INVESTING AND FINANCING ACTIVITY
Construction costs payable capitalized to real estate	41,328	32,228	28,805
Accrual of dividends and distributions	55,679	48,976	41,849
NON-CASH OPERATING ACTIVITY
Lease liabilities arising from obtaining right-of-use assets	$ 114,989	$ 8,330	$ 73,575

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2018
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, 2018, the Company owns a 75.8% common interest in our Operating Partnership, and affiliates of The Carlyle Group and others own a 24.2% 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, 2018
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. Recently Adopted Accounting Pronouncements Revenue from Contracts with Customers 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. The standard provides guidance for our nonlease revenue components. We adopted this standard effective January 1, 2018, using the cumulative effect method. The adoption did not result in a cumulative catch-up adjustment to opening equity and does not change the recognition pattern of our operating revenues. Under the standard, disclosures are required to provide information on the nature, amount, timing, and uncertainty of revenue, certain costs, and cash flows arising from contracts with customers. See additional discussion below and in Note 7, Lease Revenue. Leases In February 2016, the FASB issued guidance codified in ASC Topic 842, Leases , which amended the guidance in former ASC Topic 840, Leases . The new standard increases transparency and comparability by requiring the recognition by lessees of right-of-use (“ROU”) assets and lease liabilities on the balance sheet for those leases classified as operating leases. Under the standard, disclosures are required to meet the objective of enabling users of financial statements to assess the amount, timing, and uncertainty of cash flows arising from leases. We elected to early adopt the lease standard effective January 1, 2018, concurrent with our adoption of the new revenue recognition standard. The lease standard required a modified retrospective transition approach as of the January 1, 2016, transition date. We elected the package of practical expedients, which permits us to not reassess (1) whether any expired or existing contracts are or contain leases, (2) the lease classification for any expired or existing leases, and (3) any initial direct costs for any existing leases as of the effective date. We did not elect the hindsight practical expedient, which permits entities to use hindsight in determining the lease term and assessing impairment. The adoption of the lease standard did not result in a cumulative catch-up adjustment to opening equity. Adoption of the lease standard had a material impact on our consolidated balance sheets. As a lessee, we adjusted certain previously reported financial statements to include the recognition of ROU assets and lease liabilities for operating leases. See the table below for the impact of adoption of the lease standard on our consolidated balance sheet as of December 31, 2017 (in thousands): As Previously New Lease Standard As Adjusted Operating lease right-of-use assets $ — $ 92,984 $ 92,984 Operating lease liabilities — 102,912 102,912 Deferred rent payable 9,928 (9,928) — As a lessor, our recognition of rental revenue remained mainly consistent with previous guidance, 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 no longer 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 our consolidated statements of operations. As a result of electing the package of practical expedients described above, existing leases, including the allocation of consideration between lease and nonlease components, and related initial direct costs have not been reassessed prior to the effective date and therefore adoption of the lease standard did not have an impact on our previously reported consolidated statements of operations for initial direct costs. In July 2018, the FASB issued guidance codified in Accounting Standards Update (“ASU”) 2018-11, Leases – Targeted Improvements . The ASU provides a practical expedient, which allows lessors to combine nonlease components with the related lease components if both the timing and pattern of transfer are the same for the nonlease component(s) and related lease component, and the lease component would be classified as an operating lease. The single combined component is accounted for under ASC 842 if the lease component is the predominant component and is accounted for under ASC 606 if the nonlease components are the predominant components. Lessors are permitted to apply the practical expedient to all existing leases on a retrospective or prospective basis. We elected the practical expedient to combine our lease and nonlease components that meet the defined criteria and will account for the combined lease component under ASC 842 on a retrospective basis. As a result of electing this practical expedient from ASU 2018-11, we have adjusted our prior period consolidated statements of operations to present our data center revenues and conform to our new presentation as follows: Previous Presentation ASU 2018-11 Adjustment Adjusted Consolidated Statement of Operations Year Ended December 31, 2017 Rental revenue $ 264,134 $ (264,134) $ — Power revenue 134,909 (134,909) — Tenant reimbursement and other 8,637 (8,637) — Rental, power, and related revenue — 407,680 407,680 Year Ended December 31, 2016 Rental revenue $ 218,060 $ (218,060) $ — Power revenue 111,541 (111,541) — Tenant reimbursement and other 9,086 (9,086) — Rental, power, and related revenue — 338,687 338,687 Statement of Cash Flows 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. We adopted this standard effective January 1, 2018, and the provisions of ASU 2016-15 did not have a material impact on our consolidated financial statements. Derivatives and Hedging 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 elected to early adopt this standard effective April 1, 2018, with an initial application date of January 1, 2018, using a modified retrospective transition and the provisions of ASU 2017-12 did not have a material impact on our consolidated financial statements. Intangibles – Goodwill and Other 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 is 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 elected to early adopt this standard effective October 1, 2018, and the provisions of ASU 2017-04 did not have a material impact on our consolidated financial statements. Derivatives and Hedging In October 2018, the FASB issued guidance codified in ASU 2018-16, Derivatives and Hedging (Topic 815): Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes. ASU 2018-16 permits the use of the OIS rate based on SOFR as a U.S. benchmark interest rate for hedge accounting purposes under Topic 815. The standard will be effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted for entities that have already adopted ASU 2017-12. We elected to early adopt this standard upon issuance and the provisions of ASU 2018-16 did not have a material impact on our consolidated financial statements. Recent Accounting Pronouncements Not Yet Adopted Fair Value Measurement In August 2018, the FASB issued guidance codified in ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU 2018-13 improves the overall usefulness of disclosures to financial statement users and reduces unnecessary costs in preparing fair value measurement disclosures. 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 2018-13 to have a material impact on our consolidated financial statements. Intangibles – Goodwill and Other – Internal-Use Software In August 2018, the FASB issued guidance codified in ASU 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 clarifies that implementation costs incurred by customers in cloud computing arrangements are deferred if they would be capitalized by customers in software licensing arrangements under the internal-use software guidance. Additionally, ASU 2018-15 clarifies that all capitalized costs must be presented in the same financial statement line item as the cloud computing arrangement. The standard will be effective, on either a prospective or retrospective basis, for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. We are currently evaluating the provisions of ASU 2018-15 and whether the provisions will have a material impact on our consolidated financial statements. We determined that all other recently issued accounting pronouncements will not have a material impact on our consolidated financial statements or do not apply to our operations. 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 our standalone selling prices, performance-based equity compensation plans and the carrying values of our real estate properties, goodwill and accrued liabilities. 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 2017 and 2016 have been reclassified to conform to the 2018 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 $5.6 million, $3.3 million and $4.1 million for the years ended December 31, 2018, 2017, and 2016, respectively. Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: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 $122.9 million, $108.1 million and $89.2 million for the years ended December 31, 2018, 2017, and 2016, 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 primary difference between business combinations and asset acquisitions is that asset acquisitions require cost accumulation and allocation at a relative fair value. Acquisition costs are capitalized for asset acquisitions and expensed for business combinations. 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, 2018, 2017, or 2016. 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, 2018, and 2017, we had $40.6 million of goodwill at each date. 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, 2018, 2017, or 2016. 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. Initial Direct Costs Initial direct 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. Initial direct costs are incremental costs that would not have been incurred if the lease agreement had not been executed. These initial direct costs are capitalized and generally 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 initial direct costs related to the lease are written off to amortization expense. Amortization of initial direct costs was $15.6 million, $16.3 million, and $15.1 million for the years ended December 31, 2018, 2017, and 2016, respectively. Initial direct costs are included within other assets in the consolidated balance sheets and consisted of the following, net of amortization, as of December 31, 2018, and 2017 (in thousands): December 31, December 31, 2018 2017 Internal sales commissions $ 14,199 $ 17,402 Third party commissions 9,855 11,802 Other 597 775 Total $ 24,651 $ 29,979 Deferred Financing Costs 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 indebtedness 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, 2018, 2017, and 2016, no impairment of long-lived assets was recognized in the consolidated financial statements. 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 gain or loss on the hedging instruments as a component of accumulated other comprehensive income or loss. 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, 2018, 2017, and 2016, no impairment of internal-use software was recognized in the consolidated financial statements. Revenue Recognition Rental, Power, and Related Revenue We derive our revenues from leases with customers for data center and office and light-industrial space. Our leases include rental revenue lease components and nonlease revenue components, such as power and tenant reimbursements. We have elected to combine all of our nonlease revenue components that have the same pattern of transfer as the related operating lease component into a single combined lease component. Our leases with customers are classified as operating leases and rental revenue is recognized on a straight-line basis over the customer lease term. Occasionally, our customer leases include options to extend or terminate the lease agreements. We do not include any of these extension or termination options in a customer’s lease term for lease classification purposes or for recognizing rental revenue unless we are reasonably certain the customer will exercise these extension or termination options. The excess of rents recognized over amounts contractually due pursuant to the underlying leases is recorded as deferred rent receivable within other assets on our consolidated balance sheets. In general, we provide two power products for our data center leased space, consisting of a fixed (breakered-amperage) and a variable (sub-metered) model. Customer power arrangements are coterminous with the customer’s underlying lease and have the same pattern of transfer over the lease term and are therefore combined with lease revenue within our consolidated statements of operations. For variable power arrangements, a customer pays us variable monthly fees for the specific amount of power utilized at the current utility rates. We recognize variable power revenue each month as the uncertainty related to the consideration is resolved, as power is provided to our customers, and as our customers utilize the power. Some of our leases contain provisions under which our customers reimburse us for common area maintenance and other executory costs. These customer reimbursements are variable and are recognized in the period that the expenses are recognized. These services have the same pattern of transfer over the lease term and are also combined with lease revenue within our consolidated statements of operations. Interconnection Revenue We also derive revenue from interconnection services, which are generally contracted on a month-to-month basis cancellable by us or the customer at any time. Interconnection services are accounted for as separate contracts and are not combined with lease and power arrangements. We recognize interconnection revenue each month as these services are delivered to, and utilized by, our customers. A provision for uncollectible accounts is recorded if a receivable balance relating to contractual rent, rental revenue recorded on a straight-line basis, tenant reimbursements or other billed amounts is considered by management to be uncollectible. At December 31, 2018, and 2017, the allowance for doubtful accounts totaled $0.4 million and $1.1 million, respectively, on the consolidated balance sheets. Lessee Accounting We determine if an arrangement is a lease at inception. Our operating lease agreements are primarily for real estate space and are included within operating lease ROU assets and operating lease liabilities on the consolidated balance sheets. We elected the practical expedient to combine our lease and related nonlease components for our lessee building leases. ROU assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. ROU assets and lease liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. Our variable lease payments consist of nonlease services related to the lease. Variable lease payments are excluded from the ROU assets and lease liabilities and are recognized in the period in which the obligation for those payments is incurred. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. ROU assets also include any lease payments made and exclude lease incentives. Many of our lessee agreements include options to extend the lease, which we do not include in our minimum lease terms unless they are reasonably certain to be exercised. Rental expense for lease payments related to operating leases is recognized on a straight-line basis over the lease term. 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, 2018, and 2017, the amount included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.6 million and $1.5 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. 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 regard to our activities and our subsidiary pass‑through entities. The allocable share of taxable income is included in the income tax returns of its stockholders. We are subject to the statutory requirements of the locations in which we conduct 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, 2018, and 2017, 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 2015. Concentration of Credit Risks Our cash and cash equivalents are maintained in various financial institutions, which, at times, may exceed federally insured limi

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2018
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 have started to redevelop the existing buildings into approximately 890,000 of incremental data center NRSF across multiple phases as existing light-industrial / flex office leases expire and customers vacate. 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. We are currently constructing a turn-key data center on the acquired land parcel, which we refer to as SV8. 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 plan 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 permits and entitlements. On April 20, 2018, we acquired U.S. Colo, a carrier-neutral, network-dense colocation provider, located in Los Angeles, California, for a purchase price of $6.3 million, net of previously accrued legal expense. In connection with the U.S. Colo acquisition, we assumed a leasehold interest of 6,723 square feet at our existing LA1 facility. We also assumed a leasehold interest of 21,850 square feet at a nearby colocation data center facility, which we refer to as LA4. On January 23, 2019, we entered into a contract to acquire a 3.8-acre operating light-industrial / flex office building and land parcel located adjacent to our Santa Clara campus, for a purchase price of $26 million. Pursuant to the purchase agreement and subject to customary closing conditions, we agreed to purchase the property following the customary due diligence period. We plan to build a turn-key data center building on the acquired land parcel, which we refer to as SV9, as the existing customers vacate upon expiration of their leases and upon the receipt of necessary entitlements.

Investment in Real Estate

Investment in Real Estate	12 Months Ended
	Dec. 31, 2018
Investment in Real Estate
Investment in Real Estate	4. Investment in Real Estate The following is a summary of the properties owned or leased by market at December 31, 2018 (in thousands): Buildings and Construction in Market Land Improvements Progress Total Cost Boston $ 5,154 $ 107,596 $ 1,644 $ 114,394 Chicago (1) 5,493 113,875 14,478 133,846 Denver — 30,740 476 31,216 Los Angeles (1) 18,672 338,011 51,688 408,371 Miami 728 14,014 69 14,811 New York 2,729 152,956 33,796 189,481 Northern Virginia 22,793 346,209 94,623 463,625 San Francisco Bay 31,386 626,928 69,147 727,461 Total $ 86,955 $ 1,730,329 $ 265,921 $ 2,083,205 (1) Refer to Note 3, Acquisitions, for further detail on CH2 and U.S. Colo acquisitions. 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 23,642 300,329 50,336 374,307 San Francisco Bay 31,386 616,394 16,156 663,936 Total $ 97,258 $ 1,561,056 $ 162,903 $ 1,821,217

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2018
Other Assets.
Other Assets	5. Other Assets Our other assets consisted of the following, net of amortization and depreciation, if applicable for each line item, as of December 31, 2018, and 2017 (in thousands): December 31, December 31, 2018 2017 Deferred rent receivable $ 43,162 $ 40,038 Initial direct costs 24,651 29,979 Internal-use software 17,708 17,477 Prepaid expenses 7,090 6,770 Corporate furniture, fixtures and equipment 5,006 6,408 Deferred financing costs - revolving credit facility 2,880 957 Other 1,793 1,872 Total $ 102,290 $ 103,501 Initial direct costs are amortized as amortization expense on a straight‑line basis over the remaining lease terms of the underlying leases. The estimated amortization of initial direct costs for each of the five succeeding fiscal years is as follows (in thousands): Year Ending December 31, 2019 $ 11,874 2020 6,873 2021 2,986 2022 1,164 2023 634 Thereafter 1,120 Total $ 24,651

Leases

Leases	12 Months Ended
	Dec. 31, 2018
Leases
Leases	6. Leases As the lessee, we currently lease real estate space under noncancelable operating lease agreements for our turn-key data centers at NY1, LA1, LA4, DC1, DC2, DE1, and DE2, and our corporate headquarters located in Denver, Colorado. Our leases have remaining lease terms ranging from 1 year to 11 years, some of the leases include options to extend the leases for up to an additional 20 years. We do not include any of our renewal options in our lease terms for calculating our lease liability as the renewal options allow us to maintain operational flexibility and we are not reasonably certain we will exercise these renewal options at this time. The weighted-average remaining non-cancelable lease term for our operating leases was ten years and five years at December 31, 2018, and December 31, 2017, respectively. The weighted-average discount rate was 4.9% and 4.8% at December 31, 2018, and December 31, 2017, respectively. During the year ended December 31, 2018, we extended the term of approximately 170,000 NRSF of our existing LA1 space from July 2022 to July 2029 and expanded our LA1 facility by leasing an additional 17,238 square feet, which is being developed into turn-key data center space. As a result of this lease modification, we remeasured the lease liability and adjusted the ROU asset by $109.6 million and $109.1 million, respectively. In addition, we assumed a $5.3 million lease liability and ROU asset associated with the acquisition of U.S. Colo in April 2018. The components of lease expense were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Lease expense: Operating lease expense $ 22,927 $ 20,345 $ 19,499 Variable lease expense 4,769 3,780 3,132 Rent expense $ 27,696 $ 24,125 $ 22,631 The future minimum lease payments to be paid under noncancelable operating leases in effect at December 31, 2018, are as follows (in thousands): Operating Year Ending December 31, Leases 2019 $ 25,712 2020 26,661 2021 26,392 2022 25,995 2023 24,142 Thereafter 129,606 Total lease payments $ 258,508 Less imputed interest (55,809) Total $ 202,699

Lease Revenue

Lease Revenue	12 Months Ended
	Dec. 31, 2018
Lease Revenue
Lease Revenue	7. Lease Revenue The components of data center, office, light-industrial, and other lease revenue were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Lease revenue: Minimum lease revenue $ 390,691 $ 350,497 $ 291,141 Variable lease revenue 83,992 69,031 56,134 Total lease revenue $ 474,683 $ 419,528 $ 347,275 The future minimum lease payments to be received under noncancelable data center, office, light-industrial, and other operating leases in effect at December 31, 2018, are as follows (in thousands): Minimum Year Ending December 31, Lease Payments 2019 $ 330,483 2020 220,380 2021 163,051 2022 108,666 2023 64,056 Thereafter 132,932 Total $ 1,019,568

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt
Debt	8. Debt A summary of outstanding indebtedness as of December 31, 2018, and 2017, is as follows (in thousands): Maturity December 31, December 31, Interest Rate Date 2018 2017 Revolving credit facility 3.95% and 3.11% at December 31, 2018, and December 31, 2017, respectively April 19, 2022 $ 211,500 $ 169,500 2020 Senior unsecured term loan (1) 3.37% and 3.00% at December 31, 2018, and December 31, 2017, respectively June 24, 2020 150,000 150,000 2021 Senior unsecured term loan 3.90% and 3.06% at December 31, 2018, and December 31, 2017, respectively February 2, 2021 100,000 100,000 2022 Senior unsecured term loan (2) 3.65% and 3.04% at December 31, 2018, and December 31, 2017, respectively April 19, 2022 200,000 200,000 2023 Senior unsecured term loan (3) 4.01% at December 31, 2018 April 19, 2023 150,000 — 2023 Senior unsecured notes 4.19% at December 31, 2018, and December 31, 2017, respectively June 15, 2023 150,000 150,000 2024 Senior unsecured notes 3.91% at December 31, 2018, and December 31, 2017, respectively April 20, 2024 175,000 175,000 Total principal outstanding ` 1,136,500 944,500 Unamortized deferred financing costs (5,677) (4,930) Total debt $ 1,130,823 $ 939,570 (1) Our Operating Partnership has in place a swap agreement with respect to the 2020 Term Loan (as defined below), effective through May 5, 2020, 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 2.83% per annum at our current leverage ratio as of December 31, 2018. The interest rate on the remaining $75 million of the 2020 Term Loan is based on the London Interbank Offered Rate (“LIBOR”) plus the applicable spread. The effective interest rate as of December 31, 2018, is 3.37%. See Note 9 – Derivatives and Hedging Activities. (2) Our Operating Partnership has in place a swap agreement with respect to the 2022 Term Loan (as defined below), effective through January 31, 2019, 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 2.88% per annum at our current leverage ratio as of December 31, 2018. 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, 2018, is 3.65%. See Note 9 – Derivatives and Hedging Activities. Effective January 31, 2019, the interest rate swap was terminated. (3) Our Operating Partnership has in place a swap agreement with respect to the 2023 Term Loan (as defined below), effective through April 5, 2023, to swap the variable interest rate associated with $75 million, or 50% of the principal amount of the 2023 Term Loan, to a fixed rate of approximately 4.12% per annum at our current leverage ratio as of December 31, 2018. The interest rate on the remaining $75 million of the 2023 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of December 31, 2018, is 4.01%. See Note 9 – Derivatives and Hedging Activities. Revolving Credit Facility On April 19, 2018, our Operating Partnership and certain subsidiary co-borrowers entered into the Fourth Amended and Restated Credit Agreement (as amended and restated, the “Amended and Restated Credit Agreement”), which amended and restated our previous credit agreement, to provide additional liquidity of $250 million to the Company, which was used to pay down a portion of the then-existing revolving credit facility, fund continued development across our portfolio, and for general corporate purposes. The Amended and Restated Credit Agreement, among other things, increased the revolving credit facility from $350 million to $450 million, decreased the interest rates on borrowings under the revolving credit facility and certain term loans, and extended the maturity date from June 24, 2019, to April 19, 2022, with a one-time extension option, which, if exercised, would extend the maturity date to April 19, 2023. The exercise of the extension option is subject to the payment of an extension fee equal to 10 basis points of the total commitments under the Amended and Restated Credit Agreement at initial maturity and certain other customary conditions. The Amended and Restated Credit Agreement increased our total commitment from $600 million to $850 million, consisting of a $450 million revolving credit facility, a $150 million senior unsecured term loan scheduled to mature on June 24, 2020, a $100 million senior unsecured term loan scheduled to mature on February 2, 2021, and a new $150 million senior unsecured term loan scheduled to mature on April 19, 2023. See “2020 Senior Unsecured Term Loan,” “2021 Senior Unsecured Term Loan,” and “2023 Senior Unsecured Term Loan” below for a discussion of the $150 million, $100 million, and $150 million senior unsecured term loans, respectively. The Amended and Restated Credit Agreement also increased our accordion feature by $150 million to $350 million, which allows our Operating Partnership to increase the total commitments from $850 million to $1.2 billion, under specified circumstances, including securing capital from new or existing lenders. Borrowings under the revolving credit facility have been amended to bear interest at a variable rate per annum equal to either (i) LIBOR plus 145 basis points to 205 basis points, or (ii) a base rate plus 45 basis points to 105 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2018, our Operating Partnership’s leverage ratio was 25.3% and the interest rate was LIBOR plus 145 basis points. The total amount available for borrowing under the revolving credit facility, is equal to the lesser of $450.0 million or the availability calculated based on our unencumbered asset pool. As of December 31, 2018, the borrowing capacity was $450.0 million. As of December 31, 2018, $211.5 million was borrowed and outstanding, $4.9 million was outstanding under letters of credit, and therefore $233.6 million remained available for us to borrow under the revolving credit facility. Our ability to borrow under the Amended and Restated 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, 2018, was 25.3% ; · a maximum secured debt ratio (defined as total consolidated secured debt to total gross asset value) of 40%, which, as of December 31, 2018, was 0% ; and, · a minimum fixed charge coverage ratio (defined as adjusted consolidated earnings before interest, taxes, depreciation and amortization to consolidated fixed charges) of 1.5 to 1.0, which, as of December 31, 2018, was 7.0. to 1.0. The Amended and Restated Credit Agreement ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, and the 2024 Notes (each as defined herein) and contains the same financial covenants and other customary restrictive covenants as those debt instruments. In connection with the Amended and Restated Credit Agreement, the revolving credit facility and senior unsecured term loans were amended to remove or change certain financial covenants and other customary restrictive covenants, including removal of covenants limiting distributions (except upon an event of default), incurrence of unhedged variable rate debt, and increases or decreases, as applicable to a number of ratios and other figures in the Amended and Restated Credit Agreement resulting in increased flexibility for our Operating Partnership. As of December 31, 2018, we were in compliance with all of the financial covenants under the Amended and Restated Credit Agreement. 2020 Senior Unsecured Term Loan Pursuant to the terms of the Amended and Restated Credit Agreement, our Operating Partnership and certain subsidiaries are party to a $150 million senior unsecured term loan (the "2020 Term Loan"), which was originally entered into on June 24, 2015. 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 Term Loan, the 2023 Notes, the 2024 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2018, we were in compliance with all of the financial covenants under the 2020 Term Loan. Borrowings under the 2020 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership' s leverage ratio. At December 31, 2018, the Operating Partnership's leverage ratio was 25.3% and the interest rate was LIBOR plus 140 basis points. 2021 Senior Unsecured Term Loan On February 2, 2016, our Operating Partnership and certain subsidiaries partially exercised the accordion feature on our term loans under the Amended and Restated Credit Agreement, and entered into a $100 million senior unsecured term loan (the “2021 Term Loan”). The 2021 Term Loan is governed by the terms of the Amended and Restated Credit Agreement. 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 Term Loan, the 2023 Notes, the 2024 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2018, we were in compliance with all of the financial covenants under the 2021 Term Loan. Borrowings under the 2021 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2018, our Operating Partnership’s leverage ratio was 25.3% and the interest rate was LIBOR plus 140 basis points. 2022 Senior Unsecured Term Loan On April 19, 2017, our Operating Partnership and certain subsidiaries entered into an Amended and Restated Term Loan Agreement (as amended and restated, the “Term Loan Agreement”), which amended and restated the $100 million senior unsecured term loan, originally entered into on January 31, 2014. The Term Loan Agreement was amended and restated to, among other things, (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 (the “2024 Notes”). The 2022 Term Loan ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2024 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2018, we were in compliance with all of the financial covenants under the 2022 Term Loan. Borrowings under the 2022 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership's leverage ratio. At December 31, 2018, our Operating Partnership’s leverage ratio was 25.3% and the interest rate was LIBOR plus 140 basis points. 2023 Senior Unsecured Term Loan On April 19, 2018, pursuant to the terms of, the Amended and Restated Credit Agreement, our Operating Partnership and certain subsidiaries entered into a $150 million senior unsecured term loan (the “2023 Term Loan”). The 2023 Term Loan has a five-year term maturing on April 19, 2023. The 2023 Term Loan ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Notes, the 2024 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of December 31, 2018, we were in compliance with all of the financial covenants under the 2023 Term Loan. Borrowings under the 2023 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2018, our Operating Partnership’s leverage ratio was 25.3% and the interest rate was LIBOR plus 140 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 Amended and Restated 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 2023 Term Loan, the 2024 Notes and the Amended and Restated Credit Agreement. On June 12, 2018, the 2023 Note Purchase Agreement was amended to, among other things, conform to the same financial covenants as the Amended and Restated Credit Agreement, as described above. In addition, certain additional financial covenants in the Amended and Restated 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, 2018, 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 Term Loan, the 2023 Notes and the Amended and Restated Credit Agreement. On June 12, 2018, the 2024 Note Purchase Agreement was amended to, among other things, conform to the same financial covenants as the Amended and Restated Credit Agreement, as described above. In addition, certain additional financial covenants in the Amended and Restated 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 Amended and Restated 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, 2018, we were in compliance with all of the financial covenants under the 2024 Note Purchase Agreement. Debt Maturities The following table summarizes when our debt currently becomes due (in thousands): Year Ending December 31, 2019 $ — 2020 150,000 2021 100,000 2022 411,500 2023 300,000 Thereafter 175,000 Total principal outstanding 1,136,500 Unamortized deferred financing costs (5,677) Total debt, net $ 1,130,823

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2018
Derivatives and Hedging Activities
Derivatives and Hedging Activities	9. Derivatives and Hedging Activities On December 6, 2018, we entered into a $175 million forward starting seven-year interest rate swap agreement, effective upon issuance of future indebtedness within the terms of the agreement, to protect against adverse fluctuation in interest rates. The swap reduces our exposure to variability in cash flows relating to interest payments on $175 million of fixed or floating rate debt and effectively fixes the interest rate at approximately 2.91% per annum plus an applicable spread determined at the time of the future debt issuance. On April 19, 2018, we entered into a $75 million forward starting five-year interest rate swap agreement, effective May 5, 2018, 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 4.12% per annum. 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 2.88% per annum and 2.98% per annum for the years ending December 31, 2018, and 2017, respectively. Effective January 31, 2019, the interest rate swap was terminated. 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 2.83% and 2.93% per annum as of December 31, 2018, and 2017, respectively. All of these interest rate swap agreements were designated as effective cash flow hedges of interest rate risk 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 changes in the fair value of derivatives designated and that qualify as effective 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 loss of $3.7 million, a gain of $0.6 million, and a loss of $1.1 million for the years ended December 31, 2018, 2017, and 2016, respectively. The amounts reclassified from other comprehensive income (loss) to interest expense on the consolidated statements of operations were ($0.3) million, $0.6 million, and $1.7 million for the years ended December 31, 2018, 2017, and 2016, respectively. Total interest expense presented in the consolidated statements of operations in which the effects of cash flow hedges are recorded were $35.5 million, $24.1 million, and $12.6 million for the years ended December 31, 2018, 2017, and 2016, respectively. 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, 2019, we estimate that $0.5 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 $4.1 million and zero in derivative liabilities recognized in unearned revenue, prepaid rent and other liabilities in our consolidated balance sheets as of December 31, 2018, and 2017, respectively. We also had $1.1 million derivative assets recognized in other assets in our consolidated balance sheet as of both December 31, 2018, and 2017.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity
Stockholders' Equity	10. Stockholders’ Equity At December 31, 2018, 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, 2018, we had 36.7 million shares of common stock issued and outstanding. Since our IPO, we have issued common stock under our 2010 Plan (as defined in Note 12, Equity Incentive Award Plan), in which certain of our employees and outside directors participate to provide compensation in the form of common stock. Under the 2010 Plan, we received proceeds from the exercise of stock options of $0.3 million, $4.8 million, and $1.3 million for the years ended December 31, 2018, 2017, and 2016, 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 206,808, 329,033 and 239,068 during the years ended December 31, 2018, 2017, and 2016, respectively. See Note 12, Equity Incentive Award Plan, for additional information on our 2010 Plan. During the years ended December 31, 2018, 2017, and 2016, 11,068, 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 years ended December 31, 2018, and 2016, 2,250,000, and 3,000,000 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 2,250,000, and 3,000,000 shares of our common stock, respectively. See Note 11, Noncontrolling Interests – Operating Partnership, for additional information. Preferred Stock On December 12, 2017, the Company redeemed an aggregate of 4,600,000 shares of its 7.25% Series A Cumulative Redeemable Preferred Stock, or the 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. The Series A Preferred Stock were originally issued on December 12, 2012, 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. Dividends In 2018, 2017, and 2016, 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 2018 2017 2016 Common Stock: Ordinary income (1) $ 3.09 $ 2.13 $ 2.36 Qualified dividend — — — Capital gains — — — Return of capital 0.93 0.91 — Total dividend (2) $ 4.02 $ 3.04 $ 2.36 Preferred Stock: Ordinary income $ — $ 1.36 $ 1.81 Qualified dividend — — — Capital gains — — — Total dividend (3) $ — $ 1.36 $ 1.81 (1) All 2018 ordinary dividends are eligible for the 20% deduction generally allowable to non-corporate shareholders under Internal Revenue Code Section 199A. Shareholders are encouraged to consult with their tax advisors regarding their specific treatment of CoreSite distributions. (2) A $1.10 per share or unit distribution was paid in January, all of which will apply to tax year 2019. A $0.98 per share or unit distribution was paid in January 2018, 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 applied to tax year 2017. (3) 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. In order to comply with the REIT 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. Following passage of the Tax Cuts and Jobs Act in late 2017, beginning with tax year 2018 REIT, ordinary dividends are eligible for a 20% deduction generally allowable to non-corporate shareholders. 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, 2018, has not been filed. The taxability information presented for our dividends paid in 2018 is based upon management’s estimates.

Noncontrolling Interests - Oper

Noncontrolling Interests - Operating Partnership	12 Months Ended
	Dec. 31, 2018
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. The following table shows the common ownership interest in our Operating Partnership as of December 31, 2018, and December 31, 2017: December 31, 2018 December 31, 2017 Number of Units Percentage of Total Number of Units Percentage of Total CoreSite Realty Corporation 36,256,032 75.8 % 33,826,264 71.0 % Noncontrolling interests 11,575,268 24.2 13,836,336 29.0 Total 47,831,300 100.0 % 47,662,600 100.0 % 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, 2018, we issued 179,768 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 29, 2018, June 29, 2018, September 28, 2018, and December 31, 2018, received quarterly distributions of $0.98 per unit, $1.03 per unit, $1.03 per unit, and $1.10 per unit, respectively, payable in correlation with declared dividends on shares of our common stock. During the years ended December 31, 2018, 2017, and 2016, 2,261,068, 15,011, and 3,007,000, 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 by The Carlyle Group and other third parties of 2,261,068, 15,011, and 3,007,000 shares of our common stock, respectively. These redemptions were recorded as $21.0 million, $0.2 million, and $37.4 million reductions to noncontrolling interests in our Operating Partnership and a corresponding increase to total stockholder’s equity in the consolidated balance sheets for the years ended December 31, 2018, 2017, and 2016, respectively. The redemption value of the noncontrolling interests at December 31, 2018, was $1.0 billion based on the closing price of the Company’s common stock of $87.23 on the last trading day prior to that date.

Equity Incentive Plan

Equity Incentive Plan	12 Months Ended
	Dec. 31, 2018
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”) in 2013. 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, 2018, 2,854,253 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 ended December 31, 2013. The following table sets forth stock option activity under the 2010 Plan for the years ended December 31, 2018, 2017, and 2016: Number of Weighted Weighted Aggregate 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 Granted — — Exercised (18,656) 16.63 $ 1.6 million Forfeited — — Expired — — Options outstanding, December 31, 2018 49,133 $ 20.06 2.6 years $ 3.3 million Exercisable at December 31, 2016 249,227 $ 22.29 4.9 years $ 14.2 million 2017 67,789 $ 19.12 3.4 years $ 6.4 million 2018 49,133 $ 20.06 2.6 years $ 3.3 million As of December 31, 2017, all stock option awards were fully vested. No options were granted during the years ended December 31, 2018, 2017, or 2016. The following table sets forth the number of shares subject to options that were unvested as of December 31, 2017, and 2016, and the fair value of these options at the grant date: Number of Weighted 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 — $ — 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 Weighted 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 Granted 155,697 96.47 Forfeited (36,546) 84.10 Vested (124,115) 64.03 Unvested balance, December 31, 2018 284,879 $ 85.98 As of December 31, 2018, total unearned compensation on restricted stock awards and RSUs was approximately $16.8 million, and the weighted-average vesting period was 2.3 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 2016, 2017, and 2018. 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: The following table sets forth the number of unvested PSAs and the weighted-average fair value of these awards at the date of grant: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, 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 Granted 10,729 75,104 42,917 101.03 Performance adjustment (1) 33,403 — 16,702 — Forfeited — — — — Vested (32,003) (32,003) (32,003) 61.22 Unvested balance, December 31, 2018 50,823 173,944 112,385 $ 96.58 (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, 2018, total unearned compensation on PSAs was approximately $4.5 million, and the weighted-average vesting period was 1.9 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, 2018, 2017, and 2016: 2018 2017 2016 Expected term (in years) 2.82 2.81 2.83 Expected volatility 24.15 % 23.33 % 25.29 % Expected annual dividend (1) — — — Risk-free rate 2.24 % 1.60 % 0.97 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 Net income attributable to common shares $ 77,922 $ 62,605 $ 50,371 Weighted-average common shares outstanding - basic 34,957,244 33,792,759 32,289,414 Effect of potentially dilutive common shares: Stock options 47,338 88,399 192,464 Unvested awards 132,913 177,791 250,181 Weighted-average common shares outstanding - diluted 35,137,495 34,058,949 32,732,059 Net income per share attributable to common shares Basic $ 2.23 $ 1.85 $ 1.56 Diluted $ 2.22 $ 1.84 $ 1.54 In the calculations above, we have excluded weighted-average potentially dilutive securities of 132,749, 24,961, and 29,618 for the years ended December 31, 2018, 2017, and 2016, respectively, as their effect would have been antidilutive.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2018
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 for the year ended December 31, 2018. The safe harbor contribution was increased to 4% of the participant’s annual salary, effective January 1, 2019. 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, 2018, 2017, and 2016, our contributions were $1.2 million, $1.1 million, and $0.9 million, respectively.

Estimated Fair Value of Financi

Estimated Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2018
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, 2018, 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 $1.1 billion and $944.5 million as of December 31, 2018, and 2017, respectively, with a fair value of $1.1 billion and $946.6 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. Our lease liabilities are determined based on the estimated present value of our minimum lease payments under our lease agreements. The discount rate used to determine the lease liabilities is based on our estimated incremental borrowing rate, based on Level 3 inputs from the fair value hierarchy.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
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, 2018, we had open commitments related to construction contracts of approximately $179.1 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, and company-wide improvements that are ancillary to revenue generation. At December 31, 2018, we had open commitments related to these contracts of approximately $75.9 million of which $5.6 million is scheduled to be met during the year ended December 31, 2019. In the ordinary course of business, we are subject to claims and administrative proceedings. 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. The outcome of litigation and administrative proceedings is inherently uncertain. Therefore, if one or more legal or administrative matters are resolved against us in a reporting period for amounts in excess of management’s expectations, our financial condition, cash flows or results of operations for that reporting period could be materially adversely affected.

Quarterly Financial Information

Quarterly Financial Information (unaudited)	12 Months Ended
	Dec. 31, 2018
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, 2018, and 2017 (in thousands except share data): Three Months Ended December 31, September 30, June 30, March 31, 2018 2018 2018 2018 Revenue $ 139,146 $ 139,180 $ 136,447 $ 129,619 Operating income 35,366 34,473 36,103 36,337 Net income 25,898 25,020 27,279 28,566 Net income attributable to common shares 19,631 18,600 19,389 20,302 Net income per share attributable to common shares - basic $ 0.54 $ 0.52 $ 0.57 $ 0.60 Net income per share attributable to common shares - diluted $ 0.54 $ 0.52 $ 0.57 $ 0.59 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

Schedule III Real Estate and Ac

Schedule III Real Estate and Accumulated Depreciation	12 Months Ended
	Dec. 31, 2018
Schedule III Real Estate and Accumulated Depreciation
Schedule III Real Estate and Accumulated Depreciation	CoreSite Realty Corporation Schedule III Real Estate and Accumulated Depreciation December 31, 2018 (in thousands) Gross Amount Carried at Initial Cost Costs Capitalized December 31, 2018 Accumulated Year Building and Subsequent to Building and Depreciation at Acquired Property Name Location Land Improvements Acquisition Land Improvements Total December 31, 2018 or Leased BO1 Somerville, MA $ 6,100 $ 26,748 $ 81,546 $ 5,154 $ 109,240 $ 114,394 $ 59,181 2007 CH1 Chicago, IL 5,493 49,522 64,354 5,493 113,876 119,369 44,305 2010 CH2 Chicago, IL 4,383 — 10,094 — 14,477 14,477 — 2018 DC1 Washington, DC — 4,797 4,691 — 9,488 9,488 6,568 2010 DC2 Washington, DC — — 21,512 — 21,512 21,512 60 2017 DE1 Denver, CO — 37 29,007 — 29,044 29,044 5,600 2012 DE2 Denver, CO — 4 2,168 — 2,172 2,172 856 2012 LA1 Los Angeles, CA — 41,657 48,442 — 90,099 90,099 48,992 2010 LA2 Los Angeles, CA 18,672 94,114 163,631 18,672 257,745 276,417 73,505 2010 LA3 Los Angeles, CA 9,795 — 29,334 — 39,129 39,129 — 2010 LA4 Los Angeles, CA — 2,533 193 — 2,726 2,726 353 2018 MI1 Miami, FL 728 9,325 4,758 728 14,083 14,811 4,058 2010 NY1 New York, NY — — 41,564 — 41,564 41,564 25,654 2007 NY2 Secaucus, NJ 4,952 18,408 124,557 2,729 145,188 147,917 29,269 2013 SV1 San Jose, CA 6,863 91,719 43,009 6,863 134,728 141,591 42,923 2010 SV2 Milpitas, CA 5,086 5,046 27,992 5,086 33,038 38,124 21,734 2006 SV3 Santa Clara, CA 4,162 3,482 50,540 3,972 54,212 58,184 39,809 2007 SV4 Santa Clara, CA 4,632 3,716 98,086 4,501 101,933 106,434 45,936 2007 SV5 Santa Clara, CA 2,572 — 23,577 2,544 23,605 26,149 3,195 2007 SV6 Santa Clara, CA 4,741 — 36,758 4,825 36,674 41,499 2,453 2007 SV7 Santa Clara, CA 3,793 — 244,729 3,595 244,927 248,522 30,095 2007 SV8 Santa Clara, CA 12,158 — 54,800 — 66,958 66,958 — 2017 VA1 Reston, VA 6,903 32,939 94,055 6,903 126,994 133,897 72,106 2007 VA2 Reston, VA 5,197 — 134,228 5,539 133,886 139,425 31,138 2007 VA3 Reston, VA 8,529 26,411 101,154 4,679 131,415 136,094 1,985 2016 Reston Campus Expansion Reston, VA 5,671 17,538 — 5,672 17,537 23,209 1,009 2016 Total $ 120,430 $ 427,996 $ 1,534,779 $ 86,955 $ 1,996,250 $ 2,083,205 $ 590,784 None of our properties are encumbered. The aggregate cost of the total properties for federal income tax purposes was approximately $1,877.7 million (unaudited) at December 31, 2018. See accompanying report of independent registered public accounting firm. CoreSite Realty Corporation Schedule III Real Estate and Accumulated Depreciation (Continued) December 31, 2018 (in thousands) The following table reconciles the historical cost and accumulated depreciation of our properties for the years ended December 31, 2018, 2017, and 2016: Year Ended December 31, 2018 2017 2016 Property Balance, beginning of year $ 1,821,217 $ 1,643,576 $ 1,295,135 Additions - property acquisitions 7,474 12,158 58,149 Additions - improvements 254,514 165,483 290,292 Balance, end of year $ 2,083,205 $ 1,821,217 $ 1,643,576 Accumulated Depreciation Balance, beginning of year $ 473,141 $ 369,303 $ 284,219 Additions, net of disposals 117,643 103,838 85,084 Balance, end of year $ 590,784 $ 473,141 $ 369,303 See accompanying report of independent registered public accounting firm.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
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	Recently Adopted Accounting Pronouncements Revenue from Contracts with Customers 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. The standard provides guidance for our nonlease revenue components. We adopted this standard effective January 1, 2018, using the cumulative effect method. The adoption did not result in a cumulative catch-up adjustment to opening equity and does not change the recognition pattern of our operating revenues. Under the standard, disclosures are required to provide information on the nature, amount, timing, and uncertainty of revenue, certain costs, and cash flows arising from contracts with customers. See additional discussion below and in Note 7, Lease Revenue. Leases In February 2016, the FASB issued guidance codified in ASC Topic 842, Leases , which amended the guidance in former ASC Topic 840, Leases . The new standard increases transparency and comparability by requiring the recognition by lessees of right-of-use (“ROU”) assets and lease liabilities on the balance sheet for those leases classified as operating leases. Under the standard, disclosures are required to meet the objective of enabling users of financial statements to assess the amount, timing, and uncertainty of cash flows arising from leases. We elected to early adopt the lease standard effective January 1, 2018, concurrent with our adoption of the new revenue recognition standard. The lease standard required a modified retrospective transition approach as of the January 1, 2016, transition date. We elected the package of practical expedients, which permits us to not reassess (1) whether any expired or existing contracts are or contain leases, (2) the lease classification for any expired or existing leases, and (3) any initial direct costs for any existing leases as of the effective date. We did not elect the hindsight practical expedient, which permits entities to use hindsight in determining the lease term and assessing impairment. The adoption of the lease standard did not result in a cumulative catch-up adjustment to opening equity. Adoption of the lease standard had a material impact on our consolidated balance sheets. As a lessee, we adjusted certain previously reported financial statements to include the recognition of ROU assets and lease liabilities for operating leases. See the table below for the impact of adoption of the lease standard on our consolidated balance sheet as of December 31, 2017 (in thousands): As Previously New Lease Standard As Adjusted Operating lease right-of-use assets $ — $ 92,984 $ 92,984 Operating lease liabilities — 102,912 102,912 Deferred rent payable 9,928 (9,928) — As a lessor, our recognition of rental revenue remained mainly consistent with previous guidance, 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 no longer 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 our consolidated statements of operations. As a result of electing the package of practical expedients described above, existing leases, including the allocation of consideration between lease and nonlease components, and related initial direct costs have not been reassessed prior to the effective date and therefore adoption of the lease standard did not have an impact on our previously reported consolidated statements of operations for initial direct costs. In July 2018, the FASB issued guidance codified in Accounting Standards Update (“ASU”) 2018-11, Leases – Targeted Improvements . The ASU provides a practical expedient, which allows lessors to combine nonlease components with the related lease components if both the timing and pattern of transfer are the same for the nonlease component(s) and related lease component, and the lease component would be classified as an operating lease. The single combined component is accounted for under ASC 842 if the lease component is the predominant component and is accounted for under ASC 606 if the nonlease components are the predominant components. Lessors are permitted to apply the practical expedient to all existing leases on a retrospective or prospective basis. We elected the practical expedient to combine our lease and nonlease components that meet the defined criteria and will account for the combined lease component under ASC 842 on a retrospective basis. As a result of electing this practical expedient from ASU 2018-11, we have adjusted our prior period consolidated statements of operations to present our data center revenues and conform to our new presentation as follows: Previous Presentation ASU 2018-11 Adjustment Adjusted Consolidated Statement of Operations Year Ended December 31, 2017 Rental revenue $ 264,134 $ (264,134) $ — Power revenue 134,909 (134,909) — Tenant reimbursement and other 8,637 (8,637) — Rental, power, and related revenue — 407,680 407,680 Year Ended December 31, 2016 Rental revenue $ 218,060 $ (218,060) $ — Power revenue 111,541 (111,541) — Tenant reimbursement and other 9,086 (9,086) — Rental, power, and related revenue — 338,687 338,687 Statement of Cash Flows 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. We adopted this standard effective January 1, 2018, and the provisions of ASU 2016-15 did not have a material impact on our consolidated financial statements. Derivatives and Hedging 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 elected to early adopt this standard effective April 1, 2018, with an initial application date of January 1, 2018, using a modified retrospective transition and the provisions of ASU 2017-12 did not have a material impact on our consolidated financial statements. Intangibles – Goodwill and Other 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 is 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 elected to early adopt this standard effective October 1, 2018, and the provisions of ASU 2017-04 did not have a material impact on our consolidated financial statements. Derivatives and Hedging In October 2018, the FASB issued guidance codified in ASU 2018-16, Derivatives and Hedging (Topic 815): Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes. ASU 2018-16 permits the use of the OIS rate based on SOFR as a U.S. benchmark interest rate for hedge accounting purposes under Topic 815. The standard will be effective for interim and annual reporting periods beginning after December 15, 2018, with early adoption permitted for entities that have already adopted ASU 2017-12. We elected to early adopt this standard upon issuance and the provisions of ASU 2018-16 did not have a material impact on our consolidated financial statements. Recent Accounting Pronouncements Not Yet Adopted Fair Value Measurement In August 2018, the FASB issued guidance codified in ASU 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement. ASU 2018-13 improves the overall usefulness of disclosures to financial statement users and reduces unnecessary costs in preparing fair value measurement disclosures. 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 2018-13 to have a material impact on our consolidated financial statements. Intangibles – Goodwill and Other – Internal-Use Software In August 2018, the FASB issued guidance codified in ASU 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 clarifies that implementation costs incurred by customers in cloud computing arrangements are deferred if they would be capitalized by customers in software licensing arrangements under the internal-use software guidance. Additionally, ASU 2018-15 clarifies that all capitalized costs must be presented in the same financial statement line item as the cloud computing arrangement. The standard will be effective, on either a prospective or retrospective basis, for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted. We are currently evaluating the provisions of ASU 2018-15 and whether the provisions will have a material impact on our consolidated financial statements. We determined that all other recently issued accounting pronouncements will not have a material impact on our consolidated financial statements or do not apply to our operations.
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 our standalone selling prices, performance-based equity compensation plans and the carrying values of our real estate properties, goodwill and accrued liabilities. 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 2017 and 2016 have been reclassified to conform to the 2018 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 $5.6 million, $3.3 million and $4.1 million for the years ended December 31, 2018, 2017, and 2016, respectively. Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: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 $122.9 million, $108.1 million and $89.2 million for the years ended December 31, 2018, 2017, and 2016, 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 primary difference between business combinations and asset acquisitions is that asset acquisitions require cost accumulation and allocation at a relative fair value. Acquisition costs are capitalized for asset acquisitions and expensed for business combinations. 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, 2018, 2017, or 2016. 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, 2018, and 2017, we had $40.6 million of goodwill at each date. 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, 2018, 2017, or 2016. 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.
Initial Direct Costs	Initial Direct Costs Initial direct 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. Initial direct costs are incremental costs that would not have been incurred if the lease agreement had not been executed. These initial direct costs are capitalized and generally 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 initial direct costs related to the lease are written off to amortization expense. Amortization of initial direct costs was $15.6 million, $16.3 million, and $15.1 million for the years ended December 31, 2018, 2017, and 2016, respectively. Initial direct costs are included within other assets in the consolidated balance sheets and consisted of the following, net of amortization, as of December 31, 2018, and 2017 (in thousands): December 31, December 31, 2018 2017 Internal sales commissions $ 14,199 $ 17,402 Third party commissions 9,855 11,802 Other 597 775 Total $ 24,651 $ 29,979
Deferred Financing Costs	Deferred Financing Costs 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 indebtedness 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, 2018, 2017, and 2016, no impairment of long-lived assets was recognized in the consolidated financial statements.
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 gain or loss on the hedging instruments as a component of accumulated other comprehensive income or loss. 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, 2018, 2017, and 2016, no impairment of internal-use software was recognized in the consolidated financial statements.
Revenue Recognition	Revenue Recognition Rental, Power, and Related Revenue We derive our revenues from leases with customers for data center and office and light-industrial space. Our leases include rental revenue lease components and nonlease revenue components, such as power and tenant reimbursements. We have elected to combine all of our nonlease revenue components that have the same pattern of transfer as the related operating lease component into a single combined lease component. Our leases with customers are classified as operating leases and rental revenue is recognized on a straight-line basis over the customer lease term. Occasionally, our customer leases include options to extend or terminate the lease agreements. We do not include any of these extension or termination options in a customer’s lease term for lease classification purposes or for recognizing rental revenue unless we are reasonably certain the customer will exercise these extension or termination options. The excess of rents recognized over amounts contractually due pursuant to the underlying leases is recorded as deferred rent receivable within other assets on our consolidated balance sheets. In general, we provide two power products for our data center leased space, consisting of a fixed (breakered-amperage) and a variable (sub-metered) model. Customer power arrangements are coterminous with the customer’s underlying lease and have the same pattern of transfer over the lease term and are therefore combined with lease revenue within our consolidated statements of operations. For variable power arrangements, a customer pays us variable monthly fees for the specific amount of power utilized at the current utility rates. We recognize variable power revenue each month as the uncertainty related to the consideration is resolved, as power is provided to our customers, and as our customers utilize the power. Some of our leases contain provisions under which our customers reimburse us for common area maintenance and other executory costs. These customer reimbursements are variable and are recognized in the period that the expenses are recognized. These services have the same pattern of transfer over the lease term and are also combined with lease revenue within our consolidated statements of operations. Interconnection Revenue We also derive revenue from interconnection services, which are generally contracted on a month-to-month basis cancellable by us or the customer at any time. Interconnection services are accounted for as separate contracts and are not combined with lease and power arrangements. We recognize interconnection revenue each month as these services are delivered to, and utilized by, our customers. A provision for uncollectible accounts is recorded if a receivable balance relating to contractual rent, rental revenue recorded on a straight-line basis, tenant reimbursements or other billed amounts is considered by management to be uncollectible. At December 31, 2018, and 2017, the allowance for doubtful accounts totaled $0.4 million and $1.1 million, respectively, on the consolidated balance sheets.
Lessee Accounting	Lessee Accounting We determine if an arrangement is a lease at inception. Our operating lease agreements are primarily for real estate space and are included within operating lease ROU assets and operating lease liabilities on the consolidated balance sheets. We elected the practical expedient to combine our lease and related nonlease components for our lessee building leases. ROU assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. ROU assets and lease liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. Our variable lease payments consist of nonlease services related to the lease. Variable lease payments are excluded from the ROU assets and lease liabilities and are recognized in the period in which the obligation for those payments is incurred. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. ROU assets also include any lease payments made and exclude lease incentives. Many of our lessee agreements include options to extend the lease, which we do not include in our minimum lease terms unless they are reasonably certain to be exercised. Rental expense for lease payments related to operating leases is recognized on a straight-line basis over the lease term.
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, 2018, and 2017, the amount included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.6 million and $1.5 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. 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 regard to our activities and our subsidiary pass‑through entities. The allocable share of taxable income is included in the income tax returns of its stockholders. We are subject to the statutory requirements of the locations in which we conduct 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, 2018, and 2017, 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 2015.
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 Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Schedule of useful lives of real estate investment assets	Depreciation and amortization are calculated using the straight‑line method over the following useful lives of the assets: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
Schedule of deferred leasing costs, net of amortization	Amortization of initial direct costs was $15.6 million, $16.3 million, and $15.1 million for the years ended December 31, 2018, 2017, and 2016, respectively. Initial direct costs are included within other assets in the consolidated balance sheets and consisted of the following, net of amortization, as of December 31, 2018, and 2017 (in thousands): December 31, December 31, 2018 2017 Internal sales commissions $ 14,199 $ 17,402 Third party commissions 9,855 11,802 Other 597 775 Total $ 24,651 $ 29,979
ASU 2016-02
Schedule of financial statement impact of new accounting pronouncements	As Previously New Lease Standard As Adjusted Operating lease right-of-use assets $ — $ 92,984 $ 92,984 Operating lease liabilities — 102,912 102,912 Deferred rent payable 9,928 (9,928) —
ASU 2018-11
Schedule of financial statement impact of new accounting pronouncements	Previous Presentation ASU 2018-11 Adjustment Adjusted Consolidated Statement of Operations Year Ended December 31, 2017 Rental revenue $ 264,134 $ (264,134) $ — Power revenue 134,909 (134,909) — Tenant reimbursement and other 8,637 (8,637) — Rental, power, and related revenue — 407,680 407,680 Year Ended December 31, 2016 Rental revenue $ 218,060 $ (218,060) $ — Power revenue 111,541 (111,541) — Tenant reimbursement and other 9,086 (9,086) — Rental, power, and related revenue — 338,687 338,687

Investment in Real Estate (Tabl

Investment in Real Estate (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 (in thousands): Buildings and Construction in Market Land Improvements Progress Total Cost Boston $ 5,154 $ 107,596 $ 1,644 $ 114,394 Chicago (1) 5,493 113,875 14,478 133,846 Denver — 30,740 476 31,216 Los Angeles (1) 18,672 338,011 51,688 408,371 Miami 728 14,014 69 14,811 New York 2,729 152,956 33,796 189,481 Northern Virginia 22,793 346,209 94,623 463,625 San Francisco Bay 31,386 626,928 69,147 727,461 Total $ 86,955 $ 1,730,329 $ 265,921 $ 2,083,205 (1) Refer to Note 3, Acquisitions, for further detail on CH2 and U.S. Colo acquisitions. 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 23,642 300,329 50,336 374,307 San Francisco Bay 31,386 616,394 16,156 663,936 Total $ 97,258 $ 1,561,056 $ 162,903 $ 1,821,217

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Other Assets.
Schedule of other assets	Our other assets consisted of the following, net of amortization and depreciation, if applicable for each line item, as of December 31, 2018, and 2017 (in thousands): December 31, December 31, 2018 2017 Deferred rent receivable $ 43,162 $ 40,038 Initial direct costs 24,651 29,979 Internal-use software 17,708 17,477 Prepaid expenses 7,090 6,770 Corporate furniture, fixtures and equipment 5,006 6,408 Deferred financing costs - revolving credit facility 2,880 957 Other 1,793 1,872 Total $ 102,290 $ 103,501
Estimated amortization of deferred leasing costs for each of the five succeeding fiscal years	The estimated amortization of initial direct costs for each of the five succeeding fiscal years is as follows (in thousands): Year Ending December 31, 2019 $ 11,874 2020 6,873 2021 2,986 2022 1,164 2023 634 Thereafter 1,120 Total $ 24,651

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2018
Leases
Schedule of components of lease expense	The components of lease expense were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Lease expense: Operating lease expense $ 22,927 $ 20,345 $ 19,499 Variable lease expense 4,769 3,780 3,132 Rent expense $ 27,696 $ 24,125 $ 22,631
Schedule of future minimum lease payments to be paid under noncancelable leases	The future minimum lease payments to be paid under noncancelable operating leases in effect at December 31, 2018, are as follows (in thousands): Operating Year Ending December 31, Leases 2019 $ 25,712 2020 26,661 2021 26,392 2022 25,995 2023 24,142 Thereafter 129,606 Total lease payments $ 258,508 Less imputed interest (55,809) Total $ 202,699

Lease Revenue (Tables)

Lease Revenue (Tables)	12 Months Ended
	Dec. 31, 2018
Lease Revenue
Components of data center, office, light-industrial, and other lease revenue	The components of data center, office, light-industrial, and other lease revenue were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Lease revenue: Minimum lease revenue $ 390,691 $ 350,497 $ 291,141 Variable lease revenue 83,992 69,031 56,134 Total lease revenue $ 474,683 $ 419,528 $ 347,275
Schedule of future minimum lease payments to be received under noncancelable operating leases	The future minimum lease payments to be received under noncancelable data center, office, light-industrial, and other operating leases in effect at December 31, 2018, are as follows (in thousands): Minimum Year Ending December 31, Lease Payments 2019 $ 330,483 2020 220,380 2021 163,051 2022 108,666 2023 64,056 Thereafter 132,932 Total $ 1,019,568

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt
Summary of outstanding indebtedness	A summary of outstanding indebtedness as of December 31, 2018, and 2017, is as follows (in thousands): Maturity December 31, December 31, Interest Rate Date 2018 2017 Revolving credit facility 3.95% and 3.11% at December 31, 2018, and December 31, 2017, respectively April 19, 2022 $ 211,500 $ 169,500 2020 Senior unsecured term loan (1) 3.37% and 3.00% at December 31, 2018, and December 31, 2017, respectively June 24, 2020 150,000 150,000 2021 Senior unsecured term loan 3.90% and 3.06% at December 31, 2018, and December 31, 2017, respectively February 2, 2021 100,000 100,000 2022 Senior unsecured term loan (2) 3.65% and 3.04% at December 31, 2018, and December 31, 2017, respectively April 19, 2022 200,000 200,000 2023 Senior unsecured term loan (3) 4.01% at December 31, 2018 April 19, 2023 150,000 — 2023 Senior unsecured notes 4.19% at December 31, 2018, and December 31, 2017, respectively June 15, 2023 150,000 150,000 2024 Senior unsecured notes 3.91% at December 31, 2018, and December 31, 2017, respectively April 20, 2024 175,000 175,000 Total principal outstanding ` 1,136,500 944,500 Unamortized deferred financing costs (5,677) (4,930) Total debt $ 1,130,823 $ 939,570 (1) Our Operating Partnership has in place a swap agreement with respect to the 2020 Term Loan (as defined below), effective through May 5, 2020, 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 2.83% per annum at our current leverage ratio as of December 31, 2018. The interest rate on the remaining $75 million of the 2020 Term Loan is based on the London Interbank Offered Rate (“LIBOR”) plus the applicable spread. The effective interest rate as of December 31, 2018, is 3.37%. See Note 9 – Derivatives and Hedging Activities. (2) Our Operating Partnership has in place a swap agreement with respect to the 2022 Term Loan (as defined below), effective through January 31, 2019, 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 2.88% per annum at our current leverage ratio as of December 31, 2018. 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, 2018, is 3.65%. See Note 9 – Derivatives and Hedging Activities. Effective January 31, 2019, the interest rate swap was terminated. (3) Our Operating Partnership has in place a swap agreement with respect to the 2023 Term Loan (as defined below), effective through April 5, 2023, to swap the variable interest rate associated with $75 million, or 50% of the principal amount of the 2023 Term Loan, to a fixed rate of approximately 4.12% per annum at our current leverage ratio as of December 31, 2018. The interest rate on the remaining $75 million of the 2023 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of December 31, 2018, is 4.01%. See Note 9 – Derivatives and Hedging Activities.
Summary of amount of the entity's outstanding debt when such debt currently become due	The following table summarizes when our debt currently becomes due (in thousands): Year Ending December 31, 2019 $ — 2020 150,000 2021 100,000 2022 411,500 2023 300,000 Thereafter 175,000 Total principal outstanding 1,136,500 Unamortized deferred financing costs (5,677) Total debt, net $ 1,130,823

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Stockholders' Equity
Schedule of dividends paid per share on Series A Cumulative Preferred Stock and common shares	Year Ended December 31, Record Date 2018 2017 2016 Common Stock: Ordinary income (1) $ 3.09 $ 2.13 $ 2.36 Qualified dividend — — — Capital gains — — — Return of capital 0.93 0.91 — Total dividend (2) $ 4.02 $ 3.04 $ 2.36 Preferred Stock: Ordinary income $ — $ 1.36 $ 1.81 Qualified dividend — — — Capital gains — — — Total dividend (3) $ — $ 1.36 $ 1.81 (1) All 2018 ordinary dividends are eligible for the 20% deduction generally allowable to non-corporate shareholders under Internal Revenue Code Section 199A. Shareholders are encouraged to consult with their tax advisors regarding their specific treatment of CoreSite distributions. (2) A $1.10 per share or unit distribution was paid in January, all of which will apply to tax year 2019. A $0.98 per share or unit distribution was paid in January 2018, 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 applied to tax year 2017. (3) 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.

Noncontrolling Interests - Op_2

Noncontrolling Interests - Operating Partnership (Tables)	12 Months Ended
	Dec. 31, 2018
Noncontrolling Interests - Operating Partnership
Schedule of ownership interests in our Operating Partnership	December 31, 2018 December 31, 2017 Number of Units Percentage of Total Number of Units Percentage of Total CoreSite Realty Corporation 36,256,032 75.8 % 33,826,264 71.0 % Noncontrolling interests 11,575,268 24.2 13,836,336 29.0 Total 47,831,300 100.0 % 47,662,600 100.0 %

Equity Incentive Award Plan (Ta

Equity Incentive Award Plan (Tables)	12 Months Ended
	Dec. 31, 2018
Equity Incentive Award Plan
Schedule of stock option activity	Number of Weighted Weighted Aggregate 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 Granted — — Exercised (18,656) 16.63 $ 1.6 million Forfeited — — Expired — — Options outstanding, December 31, 2018 49,133 $ 20.06 2.6 years $ 3.3 million Exercisable at December 31, 2016 249,227 $ 22.29 4.9 years $ 14.2 million 2017 67,789 $ 19.12 3.4 years $ 6.4 million 2018 49,133 $ 20.06 2.6 years $ 3.3 million
Schedule of unvested shares subject to options	Number of Weighted 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 Weighted 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 Granted 155,697 96.47 Forfeited (36,546) 84.10 Vested (124,115) 64.03 Unvested balance, December 31, 2018 284,879 $ 85.98
Schedule of unvested PSAs and their weighted-average fair value at the date of grant	The following table sets forth the number of unvested PSAs and the weighted-average fair value of these awards at the date of grant: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, 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 Granted 10,729 75,104 42,917 101.03 Performance adjustment (1) 33,403 — 16,702 — Forfeited — — — — Vested (32,003) (32,003) (32,003) 61.22 Unvested balance, December 31, 2018 50,823 173,944 112,385 $ 96.58 (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	2018 2017 2016 Expected term (in years) 2.82 2.81 2.83 Expected volatility 24.15 % 23.33 % 25.29 % Expected annual dividend (1) — — — Risk-free rate 2.24 % 1.60 % 0.97 % The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 2017 2016 Net income attributable to common shares $ 77,922 $ 62,605 $ 50,371 Weighted-average common shares outstanding - basic 34,957,244 33,792,759 32,289,414 Effect of potentially dilutive common shares: Stock options 47,338 88,399 192,464 Unvested awards 132,913 177,791 250,181 Weighted-average common shares outstanding - diluted 35,137,495 34,058,949 32,732,059 Net income per share attributable to common shares Basic $ 2.23 $ 1.85 $ 1.56 Diluted $ 2.22 $ 1.84 $ 1.54

Quarterly Financial Informati_2

Quarterly Financial Information (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information (unaudited)
Schedule of selected quarterly information	The table below reflects the selected quarterly information for the years ended December 31, 2018, and 2017 (in thousands except share data): Three Months Ended December 31, September 30, June 30, March 31, 2018 2018 2018 2018 Revenue $ 139,146 $ 139,180 $ 136,447 $ 129,619 Operating income 35,366 34,473 36,103 36,337 Net income 25,898 25,020 27,279 28,566 Net income attributable to common shares 19,631 18,600 19,389 20,302 Net income per share attributable to common shares - basic $ 0.54 $ 0.52 $ 0.57 $ 0.60 Net income per share attributable to common shares - diluted $ 0.54 $ 0.52 $ 0.57 $ 0.59 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

Organization and Description _2

Organization and Description of Business (Details) - Operating partnership	Dec. 31, 2018	Dec. 31, 2017
Coresite Realty Corporation
Organization
Percentage of ownership interests held by the entity	75.80%	71.00%
Noncontrolling interests
Organization
Percentage of ownership interests held by the entity	75.80%
Percentage of ownership interests held by others	24.20%	29.00%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance sheet impact
Operating lease right-of-use assets	$ 190,304				$ 92,984				$ 190,304	$ 92,984
Operating lease liabilities	202,699				102,912				202,699	102,912
Income statement impact
Rental revenue									474,683	419,528	$ 347,275
Revenue	$ 139,146	$ 139,180	$ 136,447	$ 129,619	125,946	$ 123,068	$ 117,886	$ 114,921
ASU 2016-02
Balance sheet impact
Operating lease right-of-use assets					92,984					92,984
Operating lease liabilities					102,912					102,912
As Previously Reported | ASU 2016-02
Balance sheet impact
Deferred rent payable					9,928					9,928
Adjustment | ASU 2016-02
Balance sheet impact
Operating lease right-of-use assets					92,984					92,984
Operating lease liabilities					102,912					102,912
Deferred rent payable					$ (9,928)					(9,928)
Rental Revenue | As Previously Reported | ASU 2018-11
Income statement impact
Rental revenue										264,134	218,060
Rental Revenue | Adjustment | ASU 2018-11
Income statement impact
Rental revenue										(264,134)	(218,060)
Power Revenue | As Previously Reported | ASU 2018-11
Income statement impact
Revenue										134,909	111,541
Power Revenue | Adjustment | ASU 2018-11
Income statement impact
Revenue										(134,909)	(111,541)
Tenant Reimbursement And Other Revenue | As Previously Reported | ASU 2018-11
Income statement impact
Revenue										8,637	9,086
Tenant Reimbursement And Other Revenue | Adjustment | ASU 2018-11
Income statement impact
Revenue										(8,637)	(9,086)
Rental Power And Related Revenue
Income statement impact
Rental revenue									$ 463,086	407,680	338,687
Rental Power And Related Revenue | ASU 2018-11
Income statement impact
Revenue										407,680	338,687
Rental Power And Related Revenue | Adjustment | ASU 2018-11
Income statement impact
Revenue										$ 407,680	$ 338,687

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Real Estate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investments in Real Estate
Capitalized interest costs	$ 5.6	$ 3.3	$ 4.1
Depreciation expense	$ 122.9	$ 108.1	$ 89.2
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 Accoun_6

Summary of Significant Accounting Policies - Deferred Costs & Impairment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies
Impairment loss related to intangible assets	$ 0	$ 0	$ 0
Goodwill	40,646,000	40,646,000
Impairment loss related to goodwill	0	0	0
Reduction of goodwill		500,000
Deferred Costs
Amortization of initial direct costs	15,600,000	16,300,000	15,100,000
Internal sales commissions	14,199,000	17,402,000
Third party commissions	9,855,000	11,802,000
Other	597,000	775,000
Total	24,651,000	29,979,000
Recoverability of Long-Lived Assets
Impairment of real estate	$ 0	$ 0	$ 0

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Internal Use Software, RR & IT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Internal-Use Software
Recognized impairment of internal use software	$ 0	$ 0	$ 0
Revenue Recognition
Allowance for doubtful accounts (in dollars)	417	1,094
Asset Retirement and Environmental Remediation Obligations
Asset retirement and environmental remediation obligations	$ 1,600	$ 1,500
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 Accoun_8

Summary of Significant Accounting Policies - Concentration of Credit Risks & Segment Information (Details)	12 Months Ended
	Dec. 31, 2018item
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	Jan. 23, 2019USD ($)a	Apr. 20, 2018USD ($)ft²	Aug. 29, 2017USD ($)a	Nov. 10, 2016USD ($)aft²	Jan. 31, 2018USD ($)a	Jan. 29, 2018ft²
Acquisitions
Purchase price				$ 65
Area of land acquired | a				21.75
Net rentable space | ft²				890,000
SV8 Santa Clara CA
Acquisitions
Purchase price			$ 12.2
Area of land acquired | a			2
SV9 Santa Clara CA | Subsequent Event | Forecast
Acquisitions
Purchase price	$ 26
Area of land acquired | a	3.8
CH2 Chicago IL
Acquisitions
Purchase price					$ 4.5
Area of land acquired | a					2
Net rentable space | ft²						175,000
US Colo Los Angeles California
Acquisitions
Purchase price		$ 6.3
L A1 Facility Los Angeles CA
Acquisitions
Net Leased Space | ft²		6,723
LA4 Los Angeles CA
Acquisitions
Net Leased Space | ft²		21,850

Investment in Real Estate (Deta

Investment in Real Estate (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investment in Real Estate
Land	$ 86,955	$ 97,258
Buildings and Improvements	1,730,329	1,561,056
Construction in progress	265,921	162,903
Total investments in real estate	2,083,205	1,821,217
Boston
Investment in Real Estate
Land	5,154	5,154
Buildings and Improvements	107,596	106,317
Construction in progress	1,644	28
Total investments in real estate	114,394	111,499
Chicago
Investment in Real Estate
Land	5,493	5,493
Buildings and Improvements	113,875	101,798
Construction in progress	14,478	10,470
Total investments in real estate	133,846	117,761
Denver
Investment in Real Estate
Buildings and Improvements	30,740	19,193
Construction in progress	476	187
Total investments in real estate	31,216	19,380
Los Angeles
Investment in Real Estate
Land	18,672	28,467
Buildings and Improvements	338,011	273,000
Construction in progress	51,688	44,503
Total investments in real estate	408,371	345,970
Miami
Investment in Real Estate
Land	728	728
Buildings and Improvements	14,014	11,612
Construction in progress	69	28
Total investments in real estate	14,811	12,368
New York
Investment in Real Estate
Land	2,729	2,388
Buildings and Improvements	152,956	132,413
Construction in progress	33,796	41,195
Total investments in real estate	189,481	175,996
Northern Virginia
Investment in Real Estate
Land	22,793	23,642
Buildings and Improvements	346,209	300,329
Construction in progress	94,623	50,336
Total investments in real estate	463,625	374,307
San Francisco Bay
Investment in Real Estate
Land	31,386	31,386
Buildings and Improvements	626,928	616,394
Construction in progress	69,147	16,156
Total investments in real estate	$ 727,461	$ 663,936

Other Assets (Details)

Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other Assets.
Deferred rent receivable	$ 43,162	$ 40,038
Initial direct costs	24,651	29,979
Internal-use software	17,708	17,477
Prepaid expense	7,090	6,770
Corporate furniture, fixtures and equipment	5,006	6,408
Deferred financing costs - revolving credit facility	2,880	957
Other	1,793	1,872
Total	102,290	$ 103,501
Estimated amortization of deferred leasing costs
2,019	11,874
2,020	6,873
2,021	2,986
2,022	1,164
2,023	634
Thereafter	$ 1,120

Leases - (Detail)

Leases - (Detail) $ in Thousands	1 Months Ended	12 Months Ended
	Apr. 30, 2018USD ($)	Dec. 31, 2018USD ($)ft²	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Lease terms
Option to extend		P20Y
Weighted-average remaining lease term		10 years	5 years
Weighted-average discount rate		4.90%	4.80%
Increase in Right-of-Use Asset		$ 114,989	$ 8,330	$ 73,575
Lease expense
Operating lease expense		22,927	20,345	19,499
Variable lease expense		4,769	3,780	3,132
Rent expense		$ 27,696	$ 24,125	$ 22,631
Weighted-average remaining lease term		10 years	5 years
Minimum
Lease terms
Remaining lease terms		1 year
Maximum
Lease terms
Remaining lease terms		11 years
LA1
Lease terms
Net Leased Space | ft²		170,000
Increase in net rentable space | ft²		17,238
Increase in Right-of-Use Asset		$ 109,600
Increase in operating lease liability		$ 109,100
US Colo Los Angeles California
Lease terms
Increase in Right-of-Use Asset	$ 5,300

Leases - Future minimum payment

Leases - Future minimum payments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Operating leases
2,019	$ 25,712
2,020	26,661
2,021	26,392
2,022	25,995
2,023	24,142
Thereafter	129,606
Total lease payments	258,508
Less imputed interest	(55,809)
Total	202,699	$ 102,912
Operating lease right-of-use assets	$ 190,304	$ 92,984
Minimum
Operating leases
Remaining lease terms	1 year
Maximum
Operating leases
Remaining lease terms	11 years

Lease Revenue (Details)

Lease Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of lease revenue
Minimum lease revenue	$ 390,691	$ 350,497	$ 291,141
Variable lease revenue	83,992	69,031	56,134
Total lease revenue	474,683	$ 419,528	$ 347,275
Future minimum lease payments to be received under noncancelable leases
2,019	330,483
2,020	220,380
2,021	163,051
2,022	108,666
2,023	64,056
Thereafter	132,932
Total	$ 1,019,568

Debt - Summary of Debt (Details

Debt - Summary of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt
Total principal outstanding	$ 1,136,500	$ 944,500
Unamortized deferred financing costs	(5,677)	(4,930)
Total debt	$ 1,130,823	$ 939,570
Revolving Credit Facility
Debt
Interest rate as of reporting date (as a percent)	3.95%	3.11%
Total principal outstanding	$ 211,500	$ 169,500
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.37%	3.00%
Total principal outstanding	$ 150,000	$ 150,000
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.90%	3.06%
Total principal outstanding	$ 100,000	$ 100,000
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.65%	3.04%
Total principal outstanding	$ 200,000	$ 200,000
2023 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	4.01%
Total principal outstanding	$ 150,000
Amount of debt not hedged	75,000
2023 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Interest Rate Swap
Debt
Amount of debt hedged	$ 75,000
Percentage of debt hedged	50.00%
Derivative, Fixed Interest Rate	4.12%
2023 Senior Unsecured Term Loan | Senior Unsecured Notes
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 | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)	3.91%	3.91%
Total principal outstanding	$ 175,000	$ 175,000
Operating partnership | 2020 Senior Unsecured Term Loan
Debt
Amount of debt not hedged	75,000
Operating partnership | 2020 Senior Unsecured Term Loan | Interest Rate Swap
Debt
Amount of debt hedged	$ 75,000
Percentage of debt hedged	50.00%
Derivative, Fixed Interest Rate	2.83%
Operating partnership | 2022 Senior Unsecured Term Loan
Debt
Amount of debt not hedged	$ 150,000
Operating partnership | 2022 Senior Unsecured Term Loan | Interest Rate Swap
Debt
Amount of debt hedged	$ 50,000
Percentage of debt hedged	25.00%
Derivative, Fixed Interest Rate	2.88%

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility (Details) $ in Thousands	Apr. 19, 2018USD ($)item	Feb. 02, 2016USD ($)	Jun. 24, 2015USD ($)	Dec. 31, 2018USD ($)	Apr. 18, 2018USD ($)	Dec. 31, 2017USD ($)	Apr. 19, 2017USD ($)
Debt
Total principal outstanding				$ 1,136,500		$ 944,500
Revolving Credit Facility
Debt
Total principal outstanding				211,500		169,500
Credit Agreement
Debt
Letters of credit outstanding				$ 4,900
Leverage ratio (as a percent)				25.30%
Secured debt ratio, Covenant (as a percent)				0.00%
Fixed charge coverage ratio				7
Credit Agreement | Minimum
Debt
Fixed charge coverage ratio				1.5
Credit Agreement | Maximum
Debt
Leverage ratio (as a percent)				60.00%
Secured debt ratio (as a percent)				40.00%
2023 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2023 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000
2020 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000		150,000
2021 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 100,000		100,000
2022 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 200,000		$ 200,000
Operating partnership | Credit Agreement
Debt
Number of options available for extensions | item	1
Extension fee (as a percent)	0.10%
Total increase in commitment under accordion feature	$ 250,000
Maximum borrowing capacity	850,000				$ 600,000
Maximum borrowing capacity including accordion feature	1,200,000
Borrowing capacity at end of period				450,000
Amount available for borrowing under the facility				$ 233,600
Operating partnership | Credit Agreement | Revolving Credit Facility
Debt
Maximum borrowing capacity	$ 450,000				$ 350,000
Operating partnership | Credit Agreement | Revolving Credit Facility | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.45%
Operating partnership | Credit Agreement | Revolving Credit Facility | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.05%
Operating partnership | Credit Agreement | Revolving Credit Facility | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.45%
Operating partnership | Credit Agreement | Revolving Credit Facility | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.05%
Operating partnership | 2023 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2023 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	1.40%
Operating partnership | 2023 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	2.00%
Operating partnership | 2023 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	0.40%
Operating partnership | 2023 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	1.00%
Operating partnership | 2023 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Term of loan facility	5 years
Unsecured term loan	$ 150,000
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Term of loan facility			5 years
Unsecured term loan			$ 150,000
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Term of loan facility		5 years
Unsecured term loan		$ 100,000
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.50%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.10%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan							$ 100,000
Increase in accordion feature borrowing capacity							$ 300,000

Debt - Unsecured Debt (Details)

Debt - Unsecured Debt (Details) - USD ($) $ in Thousands	Apr. 19, 2018	Feb. 02, 2016	Jun. 24, 2015	Dec. 31, 2018	Dec. 31, 2017	Apr. 20, 2017	Apr. 19, 2017	Jun. 15, 2016
Debt
Total principal outstanding				$ 1,136,500	$ 944,500
2020 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000	$ 150,000
Debt Instrument, Interest Rate at Period End				3.37%	3.00%
2021 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 100,000	$ 100,000
Debt Instrument, Interest Rate at Period End				3.90%	3.06%
2022 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 200,000	$ 200,000
Debt Instrument, Interest Rate at Period End				3.65%	3.04%
2023 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)				25.30%
2023 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000
Debt Instrument, Interest Rate at Period End				4.01%
2023 Senior Unsecured Term Loan | Senior Unsecured Notes
Debt
Total principal outstanding				$ 150,000	$ 150,000
Debt Instrument, Interest Rate at Period End				4.19%	4.19%
2024 Senior Unsecured Term Loan | Senior Unsecured Notes
Debt
Total principal outstanding				$ 175,000	$ 175,000
Debt Instrument, Interest Rate at Period End				3.91%	3.91%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan			$ 150,000
Term of loan facility			5 years
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan		$ 100,000
Term of loan facility		5 years
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.50%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.10%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan							$ 100,000
Increase in accordion feature borrowing capacity							$ 300,000
Operating partnership | 2023 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2023 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	1.40%
Operating partnership | 2023 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	2.00%
Operating partnership | 2023 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	0.40%
Operating partnership | 2023 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	1.00%
Operating partnership | 2023 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan	$ 150,000
Term of loan facility	5 years
Operating partnership | 2023 Senior Unsecured Term Loan | Senior Unsecured Notes | Private Placement
Debt
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%
Operating partnership | 2024 Senior Unsecured Term Loan | Senior Unsecured Notes
Debt
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%

Debt - Maturities (Details)

Debt - Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt maturities
2,020	$ 150,000
2,021	100,000
2,022	411,500
2,023	300,000
Thereafter	175,000
Total principal outstanding	1,136,500	$ 944,500
Unamortized deferred financing costs	(5,677)	(4,930)
Total debt	$ 1,130,823	$ 939,570

Derivatives and Hedging Activ_2

Derivatives and Hedging Activities (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 06, 2018	Apr. 19, 2018	Apr. 21, 2017	Apr. 09, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative instruments and hedging activities disclosures
Amount of gain (loss) recorded in accumulated other comprehensive income						$ (3,676)	$ 580	$ (1,112)
Interest expense						35,526	24,147	12,577
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Amount of swap agreement		$ 175,000	$ 75,000	$ 100,000	$ 75,000	50,000	50,000
Term of loan facility		7 years	5 years	5 years	5 years
Derivative, Fixed Interest Rate		2.91%	4.12%
Interest rate swap terminated				$ 50,000
Amount of gain (loss) recorded in accumulated other comprehensive income						3,700	600	1,100
Amount reclassified to interest expense						(300)	600	1,700
Amount recognized in unearned revenue, prepaid rent and other liabilities						4,100	0
Interest expense						35,500	24,100	$ 12,600
Derivative asset recognized in other assets						$ 1,100	$ 1,100
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	$ 500
LIBOR | Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Reference rate			one-month LIBOR		one-month LIBOR
2020 Senior Unsecured Term Loan | Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Derivative, Fixed Interest Rate						2.83%	2.93%
2023 Senior Unsecured Term Loan | Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Derivative, Fixed Interest Rate						2.88%	2.98%

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	Dec. 12, 2017	Dec. 12, 2012	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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
Common Stock, shares issued			36,708,691	34,240,815
Common Stock, shares outstanding			36,708,691	34,240,815
Redemption of preferred stock				$ 115,000,000
Proceeds from exercise of stock options			$ 310,000	4,820,000	$ 1,290,000
2010 Plan
Stockholders' Equity
Amount of shares of common stock issued under the plan			206,808	329,033	239,068
Proceeds from exercise of stock options			$ 300,000	$ 4,800,000
Payments made for exercise of stock options					$ 1,300,000
Operating partnership
Stockholders' Equity
Number of Operating Partnership Units (in shares)			47,831,300	47,662,600
Common Operating Partnership units redeemed for common stock (in shares)			11,068	15,011	7,000
Redemption of noncontrolling interests (in shares)			2,250,000		3,000,000
Series A Cumulative Redeemable Preferred Stock
Stockholders' Equity
Accrued and unpaid dividends (in dollars per share)	$ 0.292014
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
Carlyle Group | Operating partnership
Stockholders' Equity
Common Operating Partnership units redeemed for common stock (in shares)			2,250,000		3,000,000

Stockholders' Equity - Dividend

Stockholders' Equity - Dividends (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jan. 31, 2018	Dec. 31, 2017	Jan. 31, 2017	Dec. 31, 2016
Dividends
Ordinary dividend paid (in dollars per share)			$ 0.98
Dividend deduction (as a percent)	20.00%
Subsequent Event
Dividends
Ordinary dividend paid (in dollars per share)		$ 1.10
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)	$ 3.09			$ 2.13		$ 2.36
Return of capital	0.93			0.91
Total dividend (in dollars per share)	4.02			3.04		2.36
Dividend declared taxable	$ 0.44
Ordinary dividend paid (in dollars per share)					$ 0.80
Preferred Stock
Dividends
Ordinary income (in dollars per share)				1.36		1.81
Total dividend (in dollars per share)				$ 1.36		$ 1.81
Ordinary dividend paid (in dollars per share)					$ 0.45

Noncontrolling Interests - Op_3

Noncontrolling Interests - Operating Partnership (Details) $ / shares in Units, $ in Thousands	Dec. 31, 2018USD ($)$ / sharesshares	Sep. 28, 2018$ / shares	Jun. 29, 2018$ / shares	Mar. 29, 2018$ / shares	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)shares
Noncontrolling Interests - Operating Partnership
Closing price of the Company's stock (in dollars per share) | $ / shares	$ 87.23				$ 87.23
Noncontrolling Interests
Noncontrolling Interests - Operating Partnership
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $					$ 21,002	$ 167	$ 37,374
Noncontrolling Interests | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					2,261,068	15,011	3,007,000
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $					$ 21,000	$ 200	$ 37,400
Operating partnership
Noncontrolling Interests - Operating Partnership
Conversion ratio for common operating partnership units					1
Number of Operating Partnership Units (in shares)	47,831,300				47,831,300	47,662,600
Percentage of Total	100.00%				100.00%	100.00%
Shares of common stock issued related to employee compensation arrangements					179,768
Quarterly distributions will receive by holders of operating partnership units (in dollars per unit) | $ / shares	$ 1.10	$ 1.03	$ 1.03	$ 0.98
Common Operating Partnership units redeemed for common stock (in shares)					11,068	15,011	7,000
Redemption of noncontrolling interests (in shares)					2,250,000		3,000,000
Operating partnership | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					2,250,000		3,000,000
Operating partnership | Coresite Realty Corporation
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)	36,256,032				36,256,032	33,826,264
Percentage of Total	75.80%				75.80%	71.00%
Operating partnership | Noncontrolling interests
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)	11,575,268				11,575,268	13,836,336
Percentage of Total	75.80%				75.80%
Percentage of Total	24.20%				24.20%	29.00%
Operating partnership | Noncontrolling Interests
Noncontrolling Interests - Operating Partnership
Redemption value of the noncontrolling interests | $	$ 1,000,000				$ 1,000,000

Equity Incentive Plan (Details)

Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Incentive Plan
Shares of common stock reserved for issuance	6,000,000
Shares of common stock that were available for issuance	2,854,253
Stock Options
Number of Shares Subject to Option
Options outstanding at the beginning of the period (in shares)	67,789	265,550	340,210
Exercised (in shares)	(18,656)	(197,761)	(74,660)
Options outstanding at the end of the period (in shares)	49,133	67,789	265,550	340,210
Exercisable at the end of the period (in shares)	49,133	67,789	249,227
Number of Shares Subject to Option
Unvested balance at the beginning of the period (in shares)		16,323	105,267
Vested (in shares)		(16,323)	(88,944)
Unvested balance at the end of the period (in shares)			16,323	105,267
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)		$ 9.99	$ 9.13
Vested (in dollars per share)		$ 9.99	8.97
Unvested balance at the end of the period (in dollars per share)			$ 9.99	$ 9.13
Weighted Average Remaining Contractual Term
Options outstanding at the end of the period	2 years 7 months 6 days	3 years 4 months 24 days	5 years	5 years 9 months 18 days
Exercisable at the end of the period	2 years 7 months 6 days	3 years 4 months 24 days	4 years 10 months 24 days
Aggregate Intrinsic Value
Exercised	$ 1.6	$ 13.3	$ 3.8
Outstanding at the end of the period	3.3	6.4	15	$ 11.9
Exercisable at the end of the period	$ 3.3	$ 6.4	$ 14.2
Stock Options | Weighted Average
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 19.12	$ 22.96	$ 21.77
Exercised (in dollars per share)	16.63	24.37	17.27
Options outstanding at the end of the period (in dollars per share)	20.06	19.12	22.96	$ 21.77
Exercisable at the end of the period (in dollars per share)	$ 20.06	$ 19.12	$ 22.29
Restricted Awards
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	289,843	323,641	377,612
Granted (in shares)	155,697	139,788	182,628
Forfeited (in shares)	(36,546)	(34,020)	(37,383)
Vested (in shares)	(124,115)	(139,566)	(199,216)
Unvested balance at the end of the period (in shares)	284,879	289,843	323,641	377,612
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 70.71	$ 53.84	$ 38.51
Granted (in dollars per share)	96.47	85.79	66.69
Forfeited (in dollars per share)	84.10	64	53.87
Vested (in dollars per share)	64.03	44.31	36.57
Unvested balance at the end of the period (in dollars per share)	$ 85.98	70.71	53.84	$ 38.51
Compensation cost not yet recognized
Unearned compensation	$ 16.8
Restricted Awards | Weighted Average
Compensation cost not yet recognized
Vesting period	2 years 3 months 18 days
PSAs
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 84.63	54.24	42.78
Granted (in dollars per share)	101.03	105.50	84.78
Forfeited (in dollars per share)		79.09	69.12
Vested (in dollars per share)	61.22	25.89	42.17
Unvested balance at the end of the period (in dollars per share)	$ 96.58	$ 84.63	$ 54.24	$ 42.78
Compensation cost not yet recognized
Unearned compensation	$ 4.5
Assumptions used to value the stock options granted
Expected term	2 years 9 months 26 days	2 years 9 months 22 days	2 years 9 months 29 days
Expected volatility (as a percent)	24.15%	23.33%	25.29%
Risk-free rate (as a percent)	2.24%	1.60%	0.97%
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 10 months 24 days
PSAs | Minimum
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	38,694	44,835	19,151
Granted (in shares)	10,729	7,210	11,796
Performance adjustment	33,403	62,841	29,256
Forfeited (in shares)		(5,674)	(13,474)
Vested (in shares)	(32,003)	(70,518)	(1,894)
Unvested balance at the end of the period (in shares)	50,823	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)	130,843	167,725	142,963
Granted (in shares)	75,104	50,472	82,572
Forfeited (in shares)		(16,836)	(55,916)
Vested (in shares)	(32,003)	(70,518)	(1,894)
Unvested balance at the end of the period (in shares)	173,944	130,843	167,725	142,963
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)	84,769	113,976	91,488
Granted (in shares)	42,917	28,841	47,184
Performance adjustment	16,702	23,725	11,893
Forfeited (in shares)		(11,255)	(34,695)
Vested (in shares)	(32,003)	(70,518)	(1,894)
Unvested balance at the end of the period (in shares)	112,385	84,769	113,976	91,488

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share
Net income attributable to common shares	$ 19,631	$ 18,600	$ 19,389	$ 20,302	$ 14,912	$ 15,758	$ 15,643	$ 16,292	$ 77,922	$ 62,605	$ 50,371
Weighted-average common shares outstanding - basic									34,957,244	33,792,759	32,289,414
Effect of potentially dilutive common shares:
Stock options									47,338	88,399	192,464
Unvested awards									132,913	177,791	250,181
Weighted-average common shares outstanding - diluted									35,137,495	34,058,949	32,732,059
Net income per share attributable to common shares
Basic (in dollars per share)	$ 0.54	$ 0.52	$ 0.57	$ 0.60	$ 0.44	$ 0.47	$ 0.46	$ 0.49	$ 2.23	$ 1.85	$ 1.56
Diluted (in dollars per share)	$ 0.54	$ 0.52	$ 0.57	$ 0.59	$ 0.44	$ 0.46	$ 0.46	$ 0.48	$ 2.22	$ 1.84	$ 1.54
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share									132,749	24,961	29,618

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Millions	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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		$ 1.2	$ 1.1	$ 0.9
Subsequent Event
Employee Benefit Plan
Safe harbor contribution as a percentage of employee gross pay (as a percent)	4.00%

Estimated Fair Value of Finan_2

Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Estimated Fair Value of Financial Instruments
Total principal outstanding	$ 1,136,500	$ 944,500
Significant Unobservable Inputs (Level 3)
Estimated Fair Value of Financial Instruments
Fair value of debt	$ 1,100,000	$ 946,600

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Telecommunications Capacity
Commitments and Contingencies
Open commitments		$ 75.9
Telecommunications Capacity | Forecast
Commitments and Contingencies
Open commitments met	$ 5.6
Construction Contracts
Commitments and Contingencies
Open commitments		$ 179.1

Quarterly Financial Informati_3

Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information (unaudited)
Revenue	$ 139,146	$ 139,180	$ 136,447	$ 129,619	$ 125,946	$ 123,068	$ 117,886	$ 114,921
Operating income	35,366	34,473	36,103	36,337	33,667	30,799	30,082	30,264	$ 142,279	$ 124,812	$ 94,617
Net income	25,898	25,020	27,279	28,566	27,008	24,288	24,135	25,060	106,763	100,491	81,921
Net income attributable to common shares	$ 19,631	$ 18,600	$ 19,389	$ 20,302	$ 14,912	$ 15,758	$ 15,643	$ 16,292	$ 77,922	$ 62,605	$ 50,371
Net income per share attributable to common shares - basic (in dollars per share)	$ 0.54	$ 0.52	$ 0.57	$ 0.60	$ 0.44	$ 0.47	$ 0.46	$ 0.49	$ 2.23	$ 1.85	$ 1.56
Net income per share attributable to common shares - diluted (in dollars per share)	$ 0.54	$ 0.52	$ 0.57	$ 0.59	$ 0.44	$ 0.46	$ 0.46	$ 0.48	$ 2.22	$ 1.84	$ 1.54

Schedule III Real Estate and _2

Schedule III Real Estate and Accumulated Depreciation - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Initial Cost
Land	$ 120,430
Building and Improvements	427,996
Costs Capitalized Subsequent to Acquisition	1,534,779
Gross Amount Carried As of Year End
Land	86,955
Building and Improvements	1,996,250
Total	2,083,205	$ 1,821,217	$ 1,643,576	$ 1,295,135
Accumulated Depreciation	590,784	$ 473,141	$ 369,303	$ 284,219
Aggregate cost of the total properties for federal income tax purposes	1,877,700
BO1 Somerville MA
Initial Cost
Land	6,100
Building and Improvements	26,748
Costs Capitalized Subsequent to Acquisition	81,546
Gross Amount Carried As of Year End
Land	5,154
Building and Improvements	109,240
Total	114,394
Accumulated Depreciation	59,181
CH1 Chicago IL
Initial Cost
Land	5,493
Building and Improvements	49,522
Costs Capitalized Subsequent to Acquisition	64,354
Gross Amount Carried As of Year End
Land	5,493
Building and Improvements	113,876
Total	119,369
Accumulated Depreciation	44,305
CH2 Chicago IL
Initial Cost
Land	4,383
Costs Capitalized Subsequent to Acquisition	10,094
Gross Amount Carried As of Year End
Building and Improvements	14,477
Total	14,477
DC1 Washington DC
Initial Cost
Building and Improvements	4,797
Costs Capitalized Subsequent to Acquisition	4,691
Gross Amount Carried As of Year End
Building and Improvements	9,488
Total	9,488
Accumulated Depreciation	6,568
DC2 Washington DC
Initial Cost
Costs Capitalized Subsequent to Acquisition	21,512
Gross Amount Carried As of Year End
Building and Improvements	21,512
Total	21,512
Accumulated Depreciation	60
DE1 Denver CO
Initial Cost
Building and Improvements	37
Costs Capitalized Subsequent to Acquisition	29,007
Gross Amount Carried As of Year End
Building and Improvements	29,044
Total	29,044
Accumulated Depreciation	5,600
DE2 Denver CO
Initial Cost
Building and Improvements	4
Costs Capitalized Subsequent to Acquisition	2,168
Gross Amount Carried As of Year End
Building and Improvements	2,172
Total	2,172
Accumulated Depreciation	856
LA1 Los Angeles CA
Initial Cost
Building and Improvements	41,657
Costs Capitalized Subsequent to Acquisition	48,442
Gross Amount Carried As of Year End
Building and Improvements	90,099
Total	90,099
Accumulated Depreciation	48,992
LA2 Los Angeles CA
Initial Cost
Land	18,672
Building and Improvements	94,114
Costs Capitalized Subsequent to Acquisition	163,631
Gross Amount Carried As of Year End
Land	18,672
Building and Improvements	257,745
Total	276,417
Accumulated Depreciation	73,505
LA3 Los Angeles CA
Initial Cost
Land	9,795
Costs Capitalized Subsequent to Acquisition	29,334
Gross Amount Carried As of Year End
Building and Improvements	39,129
Total	39,129
LA4 Los Angeles CA
Initial Cost
Building and Improvements	2,533
Costs Capitalized Subsequent to Acquisition	193
Gross Amount Carried As of Year End
Building and Improvements	2,726
Total	2,726
Accumulated Depreciation	353
MI1 Miami FL
Initial Cost
Land	728
Building and Improvements	9,325
Costs Capitalized Subsequent to Acquisition	4,758
Gross Amount Carried As of Year End
Land	728
Building and Improvements	14,083
Total	14,811
Accumulated Depreciation	4,058
NY1 New York NY
Initial Cost
Costs Capitalized Subsequent to Acquisition	41,564
Gross Amount Carried As of Year End
Building and Improvements	41,564
Total	41,564
Accumulated Depreciation	25,654
NY2 Secaucus NJ
Initial Cost
Land	4,952
Building and Improvements	18,408
Costs Capitalized Subsequent to Acquisition	124,557
Gross Amount Carried As of Year End
Land	2,729
Building and Improvements	145,188
Total	147,917
Accumulated Depreciation	29,269
SV1 San Jose CA
Initial Cost
Land	6,863
Building and Improvements	91,719
Costs Capitalized Subsequent to Acquisition	43,009
Gross Amount Carried As of Year End
Land	6,863
Building and Improvements	134,728
Total	141,591
Accumulated Depreciation	42,923
SV2 Milpitas CA
Initial Cost
Land	5,086
Building and Improvements	5,046
Costs Capitalized Subsequent to Acquisition	27,992
Gross Amount Carried As of Year End
Land	5,086
Building and Improvements	33,038
Total	38,124
Accumulated Depreciation	21,734
SV3 Santa Clara CA
Initial Cost
Land	4,162
Building and Improvements	3,482
Costs Capitalized Subsequent to Acquisition	50,540
Gross Amount Carried As of Year End
Land	3,972
Building and Improvements	54,212
Total	58,184
Accumulated Depreciation	39,809
SV4 Santa Clara CA
Initial Cost
Land	4,632
Building and Improvements	3,716
Costs Capitalized Subsequent to Acquisition	98,086
Gross Amount Carried As of Year End
Land	4,501
Building and Improvements	101,933
Total	106,434
Accumulated Depreciation	45,936
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	3,195
SV6 Santa Clara CA
Initial Cost
Land	4,741
Costs Capitalized Subsequent to Acquisition	36,758
Gross Amount Carried As of Year End
Land	4,825
Building and Improvements	36,674
Total	41,499
Accumulated Depreciation	2,453
SV7 Santa Clara CA
Initial Cost
Land	3,793
Costs Capitalized Subsequent to Acquisition	244,729
Gross Amount Carried As of Year End
Land	3,595
Building and Improvements	244,927
Total	248,522
Accumulated Depreciation	30,095
SV8 Santa Clara CA
Initial Cost
Land	12,158
Costs Capitalized Subsequent to Acquisition	54,800
Gross Amount Carried As of Year End
Building and Improvements	66,958
Total	66,958
VA1 Reston VA
Initial Cost
Land	6,903
Building and Improvements	32,939
Costs Capitalized Subsequent to Acquisition	94,055
Gross Amount Carried As of Year End
Land	6,903
Building and Improvements	126,994
Total	133,897
Accumulated Depreciation	72,106
VA2 Reston VA
Initial Cost
Land	5,197
Costs Capitalized Subsequent to Acquisition	134,228
Gross Amount Carried As of Year End
Land	5,539
Building and Improvements	133,886
Total	139,425
Accumulated Depreciation	31,138
VA3 Reston VA
Initial Cost
Land	8,529
Building and Improvements	26,411
Costs Capitalized Subsequent to Acquisition	101,154
Gross Amount Carried As of Year End
Land	4,679
Building and Improvements	131,415
Total	136,094
Accumulated Depreciation	1,985
Reston Campus Expansion
Initial Cost
Land	5,671
Building and Improvements	17,538
Gross Amount Carried As of Year End
Land	5,672
Building and Improvements	17,537
Total	23,209
Accumulated Depreciation	$ 1,009

Schedule III Real Estate and _3

Schedule III Real Estate and Accumulated Depreciation - Rollforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property
Balance, beginning of year	$ 1,821,217	$ 1,643,576	$ 1,295,135
Additions - property acquisitions	7,474	12,158	58,149
Additions - improvements	254,514	165,483	290,292
Balance, end of year	2,083,205	1,821,217	1,643,576
Accumulated Depreciation
Balance, beginning of year	473,141	369,303	284,219
Additions, net of disposals	117,643	103,838	85,084
Balance, end of year	$ 590,784	$ 473,141	$ 369,303