CoreSite Realty 10-K 7 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Feb. 05, 2020	Jun. 28, 2019
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2019
Entity File Number	001-34877
Entity Registrant Name	CoreSite Realty Corp
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	27-1925611
Entity Address, Address Line One	1001 17th Street, Suite 500
Entity Address, City or Town	Denver
Entity Address, State or Province	CO
Entity Address, Postal Zip Code	80202
City Area Code	866
Local Phone Number	777-2673
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	COR
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		37,694,718
Entity Public Float			$ 3,591.8
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Central Index Key	0001490892
Amendment Flag	false

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments in real estate:
Land	$ 94,593	$ 86,955
Buildings and improvements	1,989,731	1,730,329
Total investments in real estate	2,084,324	1,817,284
Less: Accumulated depreciation and amortization	(720,498)	(590,784)
Net investment in operating properties	1,363,826	1,226,500
Construction in progress	394,474	265,921
Net investments in real estate	1,758,300	1,492,421
Operating lease right-of-use assets, net	172,976	190,304
Cash and cash equivalents	3,048	2,599
Accounts and other receivables, net of allowance for doubtful accounts of $371 and $417 as of December 31, 2019, and December 31, 2018, respectively	21,008	18,464
Lease intangibles, net of accumulated amortization of $4,022 and $7,756 as of December 31, 2019, and December 31, 2018, respectively	3,939	6,943
Goodwill	40,646	40,646
Other assets, net	101,082	102,290
Total assets	2,100,999	1,853,667
Liabilities:
Debt, net of unamortized deferred financing costs of $9,098 and $5,677 as of December 31, 2019, and December 31, 2018, respectively	1,478,402	1,130,823
Operating lease liabilities	187,443	202,699
Accounts payable and accrued expenses	123,304	89,315
Accrued dividends and distributions	62,332	55,679
Acquired below-market lease contracts, net of accumulated amortization of $1,511 and $3,840 as of December 31, 2019, and December 31, 2018, respectively	2,511	2,846
Unearned revenue, prepaid rent and other liabilities	33,119	37,672
Total liabilities	1,887,111	1,519,034
Stockholders' equity:
Common Stock, par value $0.01, 100,000,000 shares authorized and 37,701,042 and 36,708,691 shares issued and outstanding at December 31, 2019, and December 31, 2018, respectively	373	363
Additional paid-in capital	512,324	491,314
Accumulated other comprehensive loss	(6,026)	(2,193)
Distributions in excess of net income	(348,509)	(246,929)
Total stockholders' equity	158,162	242,555
Noncontrolling interests	55,726	92,078
Total equity	213,888	334,633
Total liabilities and equity	$ 2,100,999	$ 1,853,667

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
CONDENSED CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts (in dollars)	$ 371	$ 417
Accumulated amortization, lease intangibles (in dollars)	4,022	7,756
Deferred financing costs - revolving credit facility	9,098	5,677
Accumulated amortization of below-market lease	$ 1,511	$ 3,840
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	37,701,042	36,708,691
Common Stock, shares outstanding	37,701,042	36,708,691

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Data center revenue:
Interconnection revenue	$ 75,751	$ 69,709	$ 62,293
Total operating revenues	572,727	544,392	481,821
Operating expenses:
Property operating and maintenance	157,293	152,357	132,820
Real estate taxes and insurance	22,866	19,239	14,913
Depreciation and amortization	152,925	141,633	129,251
Sales and marketing	22,439	21,023	18,176
General and administrative	43,764	40,090	37,548
Rent	32,624	27,696	24,125
Transaction costs		75	176
Total operating expenses	431,911	402,113	357,009
Operating income	140,816	142,279	124,812
Interest expense	(41,712)	(35,526)	(24,147)
Income before income taxes	99,104	106,753	100,665
Income tax (expense) benefit	(67)	10	(174)
Net income	99,037	106,763	100,491
Net income attributable to noncontrolling interests	23,197	28,841	25,636
Net income attributable to CoreSite Realty Corporation	75,840	77,922	74,855
Preferred stock dividends			(7,924)
Original issuance costs associated with redeemed preferred stock			(4,326)
Net income attributable to common shares	$ 75,840	$ 77,922	$ 62,605
Net income per share attributable to common shares:
Basic	$ 2.06	$ 2.23	$ 1.85
Diluted	$ 2.05	$ 2.22	$ 1.84
Weighted average common shares outstanding
Basic	36,766,310	34,957,244	33,792,759
Diluted	36,943,873	35,137,495	34,058,949
Rental, power, and related revenue
Data center revenue:
Rental revenue	$ 485,131	$ 463,086	$ 407,680
Office, light-industrial and other revenue
Data center revenue:
Rental revenue	$ 11,845	$ 11,597	$ 11,848

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 99,037	$ 106,763	$ 100,491
Other comprehensive (loss) income:
Unrealized (loss) gain on derivative contracts	(5,038)	(3,676)	580
Reclassification of other comprehensive income (loss) to interest expense	212	(326)	623
Comprehensive income	94,211	102,761	101,694
Comprehensive income attributable to noncontrolling interests	22,030	27,912	25,985
Comprehensive income attributable to common shares	$ 72,181	$ 74,849	$ 75,709

CONDENSED CONSOLIDATED STATEM_3

CONDENSED 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, 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 (loss)				854		854	349	1,203
Balance at Dec. 31, 2017		$ 338	457,495	753	(177,566)	281,020	137,624	418,644
Balance (in shares) at Dec. 31, 2017		34,240,815
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 (loss)				(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
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests		$ 8	5,414	(174)		5,248	(5,248)
Redemption of noncontrolling interests (in shares)		803,610
Issuance of stock awards, net of forfeitures (in shares)		171,354
Exercise of stock options			364			364		364
Exercise of stock options (in shares)		17,387
Share-based compensation		$ 2	15,232			15,234		15,234
Dividends and distributions					(177,420)	(177,420)	(53,134)	(230,554)
Net income					75,840	75,840	23,197	99,037
Other comprehensive income (loss)				(3,659)		(3,659)	(1,167)	(4,826)
Balance at Dec. 31, 2019		$ 373	$ 512,324	$ (6,026)	$ (348,509)	$ 158,162	$ 55,726	$ 213,888
Balance (in shares) at Dec. 31, 2019		37,701,042

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 99,037	$ 106,763	$ 100,491
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	152,925	141,633	129,251
Amortization of above/below market leases	(254)	(580)	(598)
Amortization of deferred financing costs and hedge amortization	3,338	2,370	1,676
Share-based compensation	14,384	12,038	8,946
Bad debt expense	291	(229)	1,338
Changes in operating assets and liabilities:
Accounts receivable	(2,836)	10,923	(5,087)
Deferred rent receivable	4,827	(3,124)	(3,645)
Deferred leasing costs	(13,422)	(11,672)	(13,315)
Other assets	(8,132)	(4,079)	(10,924)
Accounts payable and accrued expenses	8,208	3,502	1,224
Unearned revenue, prepaid rent and other liabilities	(9,097)	(1,275)	(2,235)
Operating leases	2,316	2,491	2,234
Net cash provided by operating activities	251,585	258,761	209,356
CASH FLOWS FROM INVESTING ACTIVITIES
Tenant improvements	(4,014)	(5,247)	(7,850)
Real estate improvements	(342,187)	(239,715)	(153,956)
Business combinations and asset acquisitions	(26,060)	(10,681)	(12,158)
Net cash used in investing activities	(372,261)	(255,643)	(173,964)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercise of stock options	364	310	4,820
Proceeds from revolving credit facility	397,298	253,036	274,000
Payments on revolving credit facility	(546,299)	(211,036)	(298,500)
Proceeds from unsecured debt	500,000	150,000	275,000
Payments of loan fees and costs	(6,337)	(5,038)	(2,410)
Redemption of preferred stock			(115,000)
Dividends and distributions	(223,901)	(193,038)	(172,484)
Net cash provided by (used in) financing activities	121,125	(5,766)	(34,574)
Net change in cash and cash equivalents	449	(2,648)	818
Cash and cash equivalents, beginning of period	2,599	5,247	4,429
Cash and cash equivalents, end of period	3,048	2,599	5,247
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest, net of capitalized amounts	31,123	32,579	22,252
Cash paid for operating lease liabilities	25,206	21,994	20,764
NON-CASH INVESTING AND FINANCING ACTIVITY
Construction costs payable capitalized to real estate	64,776	41,328	32,228
Accrual of dividends and distributions	62,332	55,679	48,976
NON-CASH OPERATING ACTIVITY
Lease liabilities arising from obtaining right-of-use assets	$ 1,040	$ 114,989	$ 8,330

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2019
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, 2019, the Company owns a 77.6% common interest in our Operating Partnership, and affiliates of The Carlyle Group and others own a 22.4% 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, 2019
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation and Basis of Presentation The accompanying consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation. Recent Accounting Pronouncements Fair Value Measurement ​ In August 2018, the FASB issued guidance codified in ASU 2018-13, Fair Value Measurement 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 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. 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 $13.6 million, $5.6 million and $3.3 million for the years ended December 31, 2019, 2018, and 2017, 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 ​ 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 rental services, 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 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, 2019, and 2018, 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, 2019, 2018, or 2017. 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 $14.2 million, $15.6 million, and $16.3 million for the years ended December 31, 2019, 2018, and 2017, 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, 2019, and 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2019 2018 Internal sales commissions ​ $ 15,064 ​ $ 14,199 ​ Third party commissions ​ ​ 10,845 ​ ​ 9,855 ​ Other ​ ​ 462 ​ ​ 597 ​ Total ​ $ 26,371 ​ $ 24,651 ​ ​ Deferred Financing Costs ​ 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, 2019, 2018, and 2017, no 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, 2019, 2018, and 2017, no 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 fixed power arrangements, a customer pays us a fixed monthly fee for a committed available amount of power. We recognize the fixed power revenue each month over the term of the lease. 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. ​ Allowance for Doubtful Accounts ​ 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, 2019, and 2018, the allowance for doubtful accounts totaled $0.4 million on the consolidated balance sheets at each date. ​ 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 right-of-use (“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, 2019, and 2018, the amount included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.7 million and $1.6 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 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, 2019, and 2018, the gross deferred income taxes were not material. We currently have no Concentration of Credit Risks Our cash and cash equivalents are maintained in various financial institutions, which, at times, may exceed federally insured limits. We have not experienced any losses in such accounts, and management believes that the Company is not exposed to any significant credit risk in this area. We have no off-balance sheet concentrations of credit risk, such as foreign exchange contracts, option contracts, or foreign currency hedging arrangements. Segment Information We manage our business as one reportable segment consisting of investments in data centers located in the United States. Although we provide services in several markets, these operations have been aggregated into one reportable segment based on the similar economic characteristics amongst all markets, including the nature of the services provided and the type of customers purchasing these services.

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2019
Acquisitions
Acquisitions	3. Acquisitions 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 162,000 NRSF data center building on the acquired land parcel, which we refer to as SV8, of which we have placed 108,000 NRSF into service during the year ended December 31, 2019. 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 are currently constructing a 169,000 NRSF turn-key data center building on the acquired land parcel, which we refer to as CH2. 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 April 12, 2019, we acquired a 3.8-acre land parcel with a single-story office building located adjacent to our Santa Clara campus, for a purchase price of $26 million. We expect to develop an approximately 200,000 NRSF turn-key data center building on the acquired land parcel, which we refer to as SV9, as the existing office tenants 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, 2019
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, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Buildings and ​ Construction in ​ ​ ​ ​ Market Land Improvements Progress Total Cost Boston ​ $ 5,154 ​ $ 119,227 ​ $ 931 ​ $ 125,312 ​ Chicago (1) ​ ​ 5,493 ​ ​ 115,699 ​ ​ 100,118 ​ ​ 221,310 ​ Denver ​ ​ — ​ ​ 32,659 ​ ​ 2,461 ​ ​ 35,120 ​ Los Angeles (1) ​ ​ 18,672 ​ ​ 376,525 ​ ​ 60,178 ​ ​ 455,375 ​ Miami ​ ​ 728 ​ ​ 14,491 ​ ​ 133 ​ ​ 15,352 ​ New York ​ ​ 2,729 ​ ​ 155,746 ​ ​ 56,271 ​ ​ 214,746 ​ Northern Virginia ​ ​ 21,856 ​ ​ 398,742 ​ ​ 101,619 ​ ​ 522,217 ​ San Francisco Bay (1) ​ ​ 39,961 ​ ​ 776,642 ​ ​ 72,763 ​ ​ 889,366 ​ Total ​ $ 94,593 ​ $ 1,989,731 ​ $ 394,474 ​ $ 2,478,798 ​ ​ (1) Refer to Note 3, Acquisitions, for further detail on CH2, U.S. Colo, SV8, and SV9 acquisitions. ​ The following is a summary of the properties owned or leased by market at December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Market Land Buildings and Construction in Total Cost ​ Boston ​ $ 5,154 ​ $ 107,596 ​ $ 1,644 ​ $ 114,394 ​ Chicago ​ ​ 5,493 ​ ​ 113,875 ​ ​ 14,478 ​ ​ 133,846 ​ Denver ​ ​ — ​ ​ 30,740 ​ ​ 476 ​ ​ 31,216 ​ Los Angeles ​ ​ 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 ​ ​

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2019
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, 2019, and 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2019 2018 Deferred rent receivable ​ $ 38,335 ​ $ 43,162 ​ Initial direct costs ​ ​ 26,371 ​ ​ 24,651 ​ Internal-use software ​ ​ 16,747 ​ ​ 17,708 ​ Prepaid expenses ​ ​ 7,675 ​ ​ 7,090 ​ Corporate furniture, fixtures and equipment ​ ​ 4,848 ​ ​ 5,006 ​ Deferred financing costs - revolving credit facility ​ ​ 3,148 ​ ​ 2,880 ​ Other ​ ​ 3,958 ​ ​ 1,793 ​ Total ​ $ 101,082 ​ $ 102,290 ​ ​ 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, ​ ​ 2020 ​ $ 10,926 ​ 2021 ​ ​ 6,722 ​ 2022 ​ ​ 3,337 ​ 2023 ​ ​ 1,506 ​ 2024 ​ ​ 1,247 ​ Thereafter ​ ​ 2,633 ​ Total ​ $ 26,371 ​ ​

Leases

Leases	12 Months Ended
	Dec. 31, 2019
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 10 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 nine years and ten years at December 31, 2019, and December 31, 2018, respectively. The weighted-average discount rate was 4.9% at each date. ​ During the year ended December 31, 2019, we extended the term of approximately 5,300 NRSF at our existing DE1 data center from October 2019 to October 2024 . As a result of this extension, we remeasured the lease liability and adjusted the ROU asset by approximately $1.0 million. ​ The components of lease expense were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ ​ 2019 ​ 2018 ​ 2017 ​ Lease expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating lease expense ​ $ 26,566 ​ $ 22,927 ​ $ 20,345 ​ Variable lease expense ​ ​ 6,058 ​ ​ 4,769 ​ ​ 3,780 ​ Rent expense ​ $ 32,624 ​ $ 27,696 ​ $ 24,125 ​ ​ ​ The future minimum lease payments to be paid under noncancelable operating leases in effect at December 31, 2019, are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ Operating ​ Year Ending December 31, ​ Leases ​ 2020 ​ $ 26,842 ​ 2021 ​ ​ 26,590 ​ 2022 ​ ​ 26,197 ​ 2023 ​ ​ 24,350 ​ 2024 ​ ​ 23,382 ​ Thereafter ​ ​ 106,361 ​ Total lease payments ​ $ 233,722 ​ Less imputed interest ​ ​ (46,279) ​ Total ​ $ 187,443 ​ ​

Lease Revenue

Lease Revenue	12 Months Ended
	Dec. 31, 2019
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, ​ ​ ​ ​ 2019 ​ 2018 ​ 2017 ​ Lease revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum lease revenue ​ ​ $ 409,622 ​ $ 390,691 ​ $ 350,497 ​ Variable lease revenue ​ ​ ​ 87,354 ​ ​ 83,992 ​ ​ 69,031 ​ Total lease revenue ​ ​ $ 496,976 ​ $ 474,683 ​ $ 419,528 ​ ​ The future minimum lease payments to be received under noncancelable data center, office, light-industrial, and other operating leases in effect at December 31, 2019, are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum ​ Year Ending December 31, Lease Payments ​ 2020 ​ ​ $ 354,857 ​ 2021 ​ ​ ​ 254,291 ​ 2022 ​ ​ ​ 170,979 ​ 2023 ​ ​ ​ 111,055 ​ 2024 ​ ​ ​ 82,759 ​ Thereafter ​ ​ ​ 212,821 ​ Total ​ ​ $ 1,186,762 ​ ​

Debt

Debt	12 Months Ended
	Dec. 31, 2019
Debt
Debt	8. Debt A summary of outstanding indebtedness as of December 31, 2019, and 2018, is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maturity ​ December 31, ​ December 31, ​ ​ Interest Rate Date 2019 2018 Revolving credit facility ​ 3.01% and 3.95% at December 31, 2019, and December 31, 2018, respectively ​ November 8, 2023 ​ $ 62,500 ​ $ 211,500 ​ 2020 Senior unsecured term loan ​ 3.37% at December 31, 2018 ​ — ​ ​ — ​ ​ 150,000 ​ 2021 Senior unsecured term loan ​ 3.90% at December 31, 2018 ​ — ​ ​ — ​ ​ 100,000 ​ 2022 Senior unsecured term loan ​ 2.96% and 3.65% at December 31, 2019, and December 31, 2018, respectively ​ April 19, 2022 ​ ​ 200,000 ​ ​ 200,000 ​ 2023 Senior unsecured notes ​ 4.19% at December 31, 2019, and December 31, 2018, respectively ​ June 15, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured term loan (1) ​ 3.44% and 4.01% at December 31, 2019, and December 31, 2018, respectively ​ April 19, 2024 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured notes ​ 3.91% at December 31, 2019, and December 31, 2018, respectively ​ April 20, 2024 ​ ​ 175,000 ​ ​ 175,000 ​ 2025 Senior unsecured term loan (1) ​ 2.81% at December 31, 2019 ​ April 1, 2025 ​ ​ 350,000 ​ ​ — ​ 2026 Senior unsecured notes (1) ​ 4.52% at December 31, 2019 ​ April 17, 2026 ​ ​ 200,000 ​ ​ — ​ 2029 Senior unsecured notes ​ 4.31% at December 31, 2019 ​ April 17, 2029 ​ ​ 200,000 ​ ​ — ​ Total principal outstanding ​ ​ ​ ` ​ ​ 1,487,500 ​ ​ 1,136,500 ​ Unamortized deferred financing costs ​ ​ ​ ​ ​ ​ (9,098) ​ ​ (5,677) ​ Total debt ​ ​ ​ ​ ​ $ 1,478,402 ​ $ 1,130,823 ​ (1) Our Operating Partnership has in place swap agreements with respect to the term loans noted above. The interest rates presented represent the effective interest rates as of December 31, 2019, including the impact of the interest rate swaps, which effectively fix the interest rate on a portion of our variable rate debt. See Note 9 – Derivatives and Hedging Activities. ​ Revolving Credit Facility On November 8, 2019, our Operating Partnership and certain subsidiary co-borrowers entered into the Fifth 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 $100 million, which was used to pay down a portion of the then-existing revolving credit facility and for general corporate purposes. The Amended and Restated Credit Agreement, among other things, decreased the interest rates on borrowings under the revolving credit facility and certain term loans, and extended the maturity date from April 19, 2022, to November 8, 2023, with a one-time extension option, which, if exercised, would extend the maturity date to November 8, 2024. The exercise of the extension option is subject to payment of an extension fee equal to 10 basis points of 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 $850 million to $950 million, consisting of a $450 million revolving credit facility, a $150 million senior unsecured term loan scheduled to mature on April 19, 2024, and a $350 million senior unsecured term loan scheduled to mature on April 1, 2025. See “2024 Senior Unsecured Term Loan” and “2025 Senior Unsecured Term Loan” below for a discussion of the $150 million and $350 million senior unsecured term loans, respectively. The Amended and Restated Credit Agreement also increased our accordion feature by $200 million to $550 million, which allows our Operating Partnership to increase our total commitments from $950 million to $1.5 billion, under specified circumstances, including secured 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 1 25 185 25 85 125 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, 2019, the borrowing capacity was $450.0 million. As of December 31, 2019, $62.5 million was borrowed and outstanding, $4.9 million was outstanding under letters of credit, and therefore $382.6 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, 2019, was 29.0% ; ● a maximum secured debt ratio (defined as total consolidated secured debt to total gross asset value) of 40%, which, as of December 31, 2019, 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, 2019 5.9 to 1.0. The Amended and Restated Credit Agreement ranks pari passu with the 2022 Term Loan, the 2024 Term Loan, the 2025 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, and the 2029 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, 2019, we were in compliance with all of the financial covenants under the Amended and Restated Credit Agreement. ​ 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 2024 Term Loan, the 2025 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, the 2029 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, 2019, 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 120 180 20 80 120 basis points. ​ 2024 Senior Unsecured Term Loan ​ ​ Borrowings under the 2024 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 120 180 20 80 120 ​ 2025 Senior Unsecured Term Loan ​ ​ Borrowings under the 2025 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 120 basis points to 180 basis points, or (ii) a base rate plus 20 basis points to 80 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2019, our Operating Partnership’s leverage ratio was 29.0% and the interest rate was LIBOR plus 120 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 ​ 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 2022 Term Loan, the 2024 Term Loan, the 2025 Term Loan, the 2024 Notes, the 2026 Notes, the 2029 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, 2019, 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 ​ 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 2022 Term Loan, the 2024 Term Loan, the 2025 Term Loan, the 2023 Notes, the 2026 Notes, the 2029 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, 2019, we were in compliance with all of the financial covenants under the 2024 Note Purchase Agreement. ​ 2026 Senior Unsecured Notes ​ On April 17, 2019, our Operating Partnership issued an aggregate principal amount of $200 million, 4.11% Series A senior unsecured notes due April 17, 2026 (the “2026 Notes”), in a private placement to certain accredited investors. After giving effect to cancellation costs incurred in connection with the termination of an interest rate swap agreement entered into in anticipation of the issuance of the Notes, the 2026 Notes bear an effective interest rate of 4.52% per annum. The terms of the 2026 Notes are governed by a note purchase agreement, dated April 17, 2019 (the “2026 Note Purchase Agreement”), by and among our Operating Partnership, the Company and the purchasers of the 2026 Notes. ​ Interest is payable semiannually, on the 15 th ​ Our Operating Partnership may prepay all or a portion of the 2026 Notes upon notice to the holders for 100% of the principal amount plus a make-whole premium as set forth in the 2026 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2026 Notes would have the right to require our Operating Partnership to purchase 100% of such holders’ 2026 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 2026 Notes rank pari passu with the 2022 Term Loan, the 2024 Term Loan, the 2025 Term Loan, the 2023 Notes, the 2024 Notes, the 2029 Notes and the Amended and Restated Credit Agreement. The 2026 Note Purchase Agreement conforms to the same financial covenants as the Amended and Restated Credit Agreement, as described above. In addition, on the date of the 2026 Note Purchase Agreement and from time to time, certain additional financial covenants in the Amended and Restated Credit Agreement will be automatically incorporated into the 2026 Note Purchase Agreement and, subject to certain conditions, 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 2026 Note Purchase Agreement. As of December 31, 2019, we were in compliance with all of the financial covenants under the 2026 Note Purchase Agreement. ​ 2029 Senior Unsecured Notes ​ As mentioned above, on April 17, 2019, our Operating Partnership entered into the 2026 Note Purchase Agreement to issue the 2026 Notes and an additional aggregate principal amount of $200 million, 4.31% Series B senior unsecured notes due April 17, 2029 (the “2029 Notes”), in a private placement to certain accredited investors. ​ Interest is payable semiannually, on the 15 th ​ Our Operating Partnership may prepay all or a portion of the 2029 Notes upon notice to the holders for 100% of the principal amount plus a make-whole premium as set forth in the 2026 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2029 Notes would have the right to require our Operating Partnership to purchase 100% of such holders’ 2029 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 2029 Notes rank pari passu with the 2022 Term Loan, the 2024 Term Loan, the 2025 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, and the Amended and Restated Credit Agreement. As of December 31, 2019, we were in compliance with all of the financial covenants under the 2026 Note Purchase Agreement. ​ Debt Maturities The following table summarizes when our debt currently becomes due (in thousands): ​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2020 ​ $ — ​ 2021 ​ ​ — ​ 2022 ​ ​ 200,000 ​ 2023 ​ ​ 212,500 ​ 2024 ​ ​ 325,000 ​ Thereafter ​ ​ 750,000 ​ Total principal outstanding ​ ​ 1,487,500 ​ Unamortized deferred financing costs ​ ​ (9,098) ​ Total debt, net ​ $ 1,478,402 ​ ​

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2019
Derivatives and Hedging Activities
Derivatives and Hedging Activities	9. Derivatives and Hedging Activities The following table summarizes our derivative positions as of the years ended December 31, 2019, and 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Notional Amount ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value (Level 2) (1) ​ December 31, ​ December 31, ​ Type of ​ Strike Rate ​ ​ Effective ​ Expiration ​ December 31, ​ December 31, ​ 2019 ​ 2018 ​ Derivative ​ as of 12/31/19 ​ ​ Date ​ Date ​ 2019 ​ 2018 ​ $ — ​ $ 50,000 ​ Interest Rate Swap ​ — % ​ 2/28/2014 ​ 1/31/2019 ​ $ — ​ $ 43 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 2.63 ​ ​ 5/5/2015 ​ 5/5/2020 ​ ​ 67 ​ ​ 1,099 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 3.92 ​ ​ 5/5/2018 ​ 4/5/2023 ​ ​ (2,819) ​ ​ (897) ​ ​ — ​ ​ 175,000 ​ Interest Rate Swap ​ — ​ ​ 6/3/2019 ​ 4/3/2019 ​ ​ — ​ ​ (3,183) ​ ​ 100,000 ​ ​ — ​ Interest Rate Swap ​ 2.79 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ 55 ​ ​ — ​ ​ 75,000 ​ ​ — ​ Interest Rate Swap ​ 2.79 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ 41 ​ ​ — ​ $ 325,000 ​ $ 375,000 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ (2,656) ​ $ (2,938) ​ (1) Derivative assets are recorded at fair value in our condensed consolidated balance sheets in other assets and derivative liabilities are recorded at fair value in our condensed consolidated balance sheets in unearned revenue, prepaid rent and other liabilities. We do not net our derivative position by counterparty for purposes of balance sheet presentation and disclosure. ​ 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 following table shows the effect of the derivatives on our consolidated financial statements (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2019 ​ 2018 ​ 2017 ​ Cash Flow Hedges ​ ​ ​ ​ ​ ​ ​ ​ ​ Unrealized gain (loss) recognized in other comprehensive income $ (5,038) ​ $ (3,676) ​ $ 580 ​ Gain (loss) reclassified from other comprehensive income to interest expense ​ 212 ​ ​ (326) ​ ​ 623 ​ Interest expense ​ 41,712 ​ ​ 35,526 ​ ​ 24,147 ​ ​ 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, 2020, we estimate that $1.6 million will be reclassified as an increase to interest expense.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2019
Stockholders' Equity
Stockholders' Equity	10. Stockholders’ Equity At December 31, 2019, 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, 2019, we had 37.7 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 $0.4 million, $0.3 million, and $4.8 million for the years ended December 31, 2019, 2018, and 2017, 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 188,741, 206,808 and 329,033 during the years ended December 31, 2019, 2018, and 2017, respectively. See Note 12, Equity Incentive Award Plan, for additional information on our 2010 Plan. During the years ended December 31, 2019, 2018, and 2017, 800,000, 2,250,000 and 3,000,000 common Operating Partnership units held by the Carlyle Group were redeemed on a one-for-one basis for shares of our common stock in connection with the sale by The Carlyle Group of our common stock. During the years ended December 31, 2019, 2018, and 2017, 3,610, 11,068 and 15,011 common Operating Partnership units held by other third parties were redeemed on a one-for-one basis for 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 2019, 2018, and 2017, 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 ​ 2019 ​ 2018 ​ 2017 ​ Common Stock: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Ordinary income (1) ​ $ 3.07 ​ $ 3.09 ​ $ 2.13 ​ Qualified dividend ​ ​ — ​ ​ — ​ ​ — ​ Capital gains ​ ​ — ​ ​ — ​ ​ — ​ Return of capital ​ ​ 1.57 ​ ​ 0.93 ​ ​ 0.91 ​ Total dividend (2) ​ $ 4.64 ​ $ 4.02 ​ $ 3.04 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Preferred Stock: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Ordinary income ​ $ — ​ $ — ​ $ 1.36 ​ Qualified dividend ​ ​ — ​ ​ — ​ ​ — ​ Capital gains ​ ​ — ​ ​ — ​ ​ — ​ Total dividend (3) ​ $ — ​ $ — ​ $ 1.36 ​ (1) All 2018 and 2019 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 $1.22 per share or unit distribution was paid in January, all of which will apply to tax year 2020. A $1.10 per share or unit distribution was paid in January 2019, 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 applied 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 generally 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, 2019 has not been filed. The taxability information presented for our dividends paid in 2019 is based upon management’s estimates.

Noncontrolling Interests - Oper

Noncontrolling Interests - Operating Partnership	12 Months Ended
	Dec. 31, 2019
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 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ December 31, 2018 ​ ​ Number of Units Percentage of Total ​ Number of Units Percentage of Total ​ CoreSite Realty Corporation ​ 37,244,987 ​ 77.6 % 36,256,032 ​ 75.8 % Noncontrolling interests ​ 10,771,658 ​ 22.4 ​ 11,575,268 ​ 24.2 ​ Total ​ 48,016,645 ​ 100.0 % 47,831,300 ​ 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, 2019, we issued 185,345 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, 2019, June 28, 2019, September 30, 2019, and December 31, 2019, received quarterly distributions of $1.10 per unit, $1.22 per unit, $1.22 per unit, and $1.22 per unit, respectively, payable in correlation with declared dividends on shares of our common stock. During the years ended December 31, 2019, 2018, and 2017, 803,610, 2,261,068, and 15,011, common Operating Partnership units held by The Carlyle Group and other third parties were redeemed on a one-for-one basis for shares of our common stock in connection with the sale by The Carlyle Group and other third parties of shares of our common stock, respectively. These redemptions were recorded as $5.2 million, $21.0 million, and $0.2 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, 2019, 2018, and 2017, respectively. The redemption value of the noncontrolling interests at December 31, 2019, was $1.2 billion based on the closing price of the Company’s common stock of $112.12 on the last trading day prior to that date.

Equity Incentive Plan

Equity Incentive Plan	12 Months Ended
	Dec. 31, 2019
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, 2019, 2,674,745 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, 2019, 2018, and 2017 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted ​ Weighted ​ Aggregate 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 ​ Granted ​ — ​ ​ — ​ ​ ​ ​ ​ Exercised ​ (17,387) ​ ​ 20.94 ​ ​ ​ $ 1.7 million ​ Forfeited ​ — ​ ​ — ​ ​ ​ ​ ​ Expired ​ — ​ ​ — ​ ​ ​ ​ ​ Options outstanding, December 31, 2019 ​ 31,746 ​ $ 19.57 ​ 1.4 years ​ $ 2.9 million ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of December 31, 2017, all stock option awards were fully vested and exercisable. ​ 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, 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 ​ Granted ​ 160,361 ​ ​ 99.91 ​ Forfeited ​ (21,910) ​ ​ 94.03 ​ Vested ​ (130,957) ​ ​ 78.32 ​ Unvested balance, December 31, 2019 ​ 292,373 ​ $ 96.44 ​ ​ As of December 31, 2019, total unearned compensation on restricted stock awards and RSUs was approximately $19.1 million, and the weighted-average vesting period was 2.4 years. Performance Stock Awards We grant long-term incentives to members of management in the form of performance-based restricted stock awards (“PSAs”) under the 2010 Plan. The number of PSAs earned is based on our achievement of relative total shareholder return (“TSR”) measured versus the MSCI US REIT Index over a three-year performance period and ranges between 25% and 175% of the target number of shares for PSAs granted in 2017, 2018, and 2019. The PSAs are granted at the maximum percentage of target and are retired annually to the extent we do not meet the maximum relative TSR performance threshold versus the MSCI US REIT Index. The PSAs are earned upon TSR achievement measured both annually and over the full three-year performance period. The PSAs have a service condition and will be released at the end of the three-year performance period, to the extent earned, provided that the holder continues to be employed or otherwise in service of the Company at the end of the three-year performance period. The PSAs are amortized on a straight-line basis to expense over the vesting period. Holders of the PSAs are entitled to dividends on the PSAs, which are accrued and paid in cash at the end of the three-year performance period. The following table sets forth the number of unvested PSAs and the weighted-average fair value of these awards at the date of grant: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ ​ ​ ​ Average Fair ​ ​ ​ Performance-Based Restricted Stock Awards ​ Value at Grant ​ ​ Minimum ​ Maximum Target Date Unvested balance, December 31, 2018 ​ 50,823 ​ 173,944 ​ 112,385 ​ $ 96.58 ​ Granted ​ 10,926 ​ 76,480 ​ 43,703 ​ ​ 114.55 ​ Performance adjustment (1) ​ 18,063 ​ (29,819) ​ (5,878) ​ ​ — ​ Forfeited ​ (1,538) ​ (5,604) ​ (3,571) ​ ​ 103.26 ​ Vested ​ (43,650) ​ (43,650) ​ (43,650) ​ ​ 84.78 ​ Unvested balance, December 31, 2019 ​ 34,624 ​ 171,351 ​ 102,989 ​ $ 107.84 ​ (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, 2019, total unearned compensation on PSAs was approximately $5.6 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, 2019, 2018, and 2017: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2019 2018 2017 Expected term (in years) ​ 2.82 ​ ​ 2.82 ​ 2.81 ​ Expected volatility ​ 24.09 % ​ 24.15 % 23.33 % Expected annual dividend (1) ​ — ​ ​ — ​ — ​ Risk-free rate ​ 2.48 % ​ 2.24 % 1.60 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2019
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, ​ ​ 2019 2018 2017 Net income attributable to common shares ​ $ 75,840 ​ $ 77,922 ​ $ 62,605 ​ Weighted-average common shares outstanding - basic ​ ​ 36,766,310 ​ ​ 34,957,244 ​ ​ 33,792,759 ​ Effect of potentially dilutive common shares: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options ​ ​ 36,685 ​ ​ 47,338 ​ ​ 88,399 ​ Unvested awards ​ ​ 140,878 ​ ​ 132,913 ​ ​ 177,791 ​ Weighted-average common shares outstanding - diluted ​ ​ 36,943,873 ​ ​ 35,137,495 ​ ​ 34,058,949 ​ Net income per share attributable to common shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ $ 2.06 ​ $ 2.23 ​ $ 1.85 ​ Diluted ​ $ 2.05 ​ $ 2.22 ​ $ 1.84 ​ ​ In the calculations above, we have excluded weighted-average potentially dilutive securities of 2,546, 132,749, and 24,961 for the years ended December 31, 2019, 2018, and 2017, respectively, as their effect would have been antidilutive.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2019
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 on the first of the month following the date of hire. Additionally, on the first of the month following 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, 2019, 2018, and 2017, our contributions were $1.7 million, $1.21 million, and $1.1 million, respectively.

Estimated Fair Value of Financi

Estimated Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2019
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 Level 2 Level 3 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, 2019, 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 $1.5 billion and $1.1 billion as of December 31, 2019 and 2018, respectively, which approximates the fair value based on Level 3 inputs from the fair value hierarchy. Under the discounted cash flow method, the fair values of the revolving credit facility, the senior unsecured term loans, and the senior unsecured notes 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 at least commencement. The discount rate used to determine the lease liabilities is based on our estimated incremental borrowing rate at lease commencement, based on Level 3 inputs from the fair value hierarchy.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
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, 2019, we had open commitments related to construction contracts of approximately $189.4 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, 2019, we had open commitments related to these contracts of approximately $74.3 million of which $6.5 million is scheduled to be met during the year ended December 31, 2020. On April 27, 2018, we filed suit against DGEB Management, LLC and its affiliates, the landlord of our DE1 facility (the “Landlord”), in the District Court, City and County of Denver, State of Colorado (the “District Court”), for certain claims relating to the building where our DE1 facility is located. The parties asserted various claims and counterclaims related to three primary areas: (1) a construction project within the DE1 building to house electrical transformers used to supply power, (2) our usage of images of the DE1 building on our website, and (3) the protocol for cross-connections within the DE1 building. The Landlord demanded approximately $21 million in damages, which included approximately $2.8 million related to the construction project, and further sought to hold us in default under the lease. On August 22, 2019, the jury found in our favor on all claims and counterclaims on which it was asked to decide, and awarded us nominal damages. Upon unopposed application, the District Court vacated deadlines for post-trial matters, including our application for reimbursement of legal fees under the lease, to provide time for the parties to discuss settlement of these post-trial matters. On December 20, 2019, the parties entered into a settlement agreement pursuant to which, among other matters, the parties agreed to file a joint motion to dismiss the litigation with prejudice and the Landlord agreed to pay us $3.1 million through delivery of a promissory note bearing interest at a floating rate of prime plus 2% per annum payable monthly. The promissory note is secured by our lease and other payment obligations, is fully pre-payable without penalty and is due December 20, 2024. ​ In the ordinary course of business, we are subject to claims and administrative proceedings. Except as described above, 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, 2019
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, 2019, and 2018 (in thousands except share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ ​ December 31, ​ September 30, ​ June 30, ​ March 31, ​ ​ 2019 2019 2019 2019 Revenue ​ $ 146,035 ​ $ 144,891 ​ $ 142,906 ​ $ 138,895 ​ Operating income ​ ​ 35,684 ​ ​ 33,643 ​ ​ 36,056 ​ ​ 35,433 ​ Net income ​ ​ 24,745 ​ ​ 22,644 ​ ​ 25,743 ​ ​ 25,905 ​ Net income attributable to common shares ​ ​ 19,194 ​ ​ 17,450 ​ ​ 19,535 ​ ​ 19,661 ​ Net income per share attributable to common shares - basic ​ $ 0.51 ​ $ 0.47 ​ $ 0.54 ​ $ 0.54 ​ Net income per share attributable to common shares - diluted ​ $ 0.51 ​ $ 0.47 ​ $ 0.53 ​ $ 0.54 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ ​

Schedule III Real Estate and Ac

Schedule III Real Estate and Accumulated Depreciation	12 Months Ended
	Dec. 31, 2019
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, 2019 (in thousands) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Amount Carried at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Initial Cost ​ Costs Capitalized ​ December 31, 2019 ​ Accumulated ​ Year ​ ​ ​ ​ ​ ​ ​ ​ Building and ​ Subsequent to ​ ​ ​ Building and ​ ​ ​ ​ Depreciation at ​ Acquired ​ Property Name Location Land Improvements Acquisition Land Improvements Total December 31, 2019 or Leased BO1 ​ Somerville, MA ​ $ 6,100 ​ $ 26,748 ​ $ 92,464 ​ $ 5,154 ​ $ 120,158 ​ $ 125,312 ​ $ 66,223 ​ 2007 ​ CH1 ​ Chicago, IL ​ ​ 5,493 ​ ​ 49,522 ​ ​ 66,321 ​ ​ 5,493 ​ ​ 115,843 ​ ​ 121,336 ​ ​ 52,821 ​ 2010 ​ CH2 ​ Chicago, IL ​ ​ 4,383 ​ ​ — ​ ​ 95,591 ​ ​ — ​ ​ 99,974 ​ ​ 99,974 ​ ​ — ​ 2018 ​ DC1 ​ Washington, DC ​ ​ — ​ ​ 4,797 ​ ​ 5,311 ​ ​ — ​ ​ 10,108 ​ ​ 10,108 ​ ​ 7,404 ​ 2010 ​ DC2 ​ Washington, DC ​ ​ — ​ ​ — ​ ​ 22,560 ​ ​ — ​ ​ 22,560 ​ ​ 22,560 ​ ​ 2,336 ​ 2017 ​ DE1 ​ Denver, CO ​ ​ — ​ ​ 37 ​ ​ 32,647 ​ ​ — ​ ​ 32,684 ​ ​ 32,684 ​ ​ 9,238 ​ 2012 ​ DE2 ​ Denver, CO ​ ​ — ​ ​ 4 ​ ​ 2,432 ​ ​ — ​ ​ 2,436 ​ ​ 2,436 ​ ​ 1,081 ​ 2012 ​ LA1 ​ Los Angeles, CA ​ ​ — ​ ​ 41,657 ​ ​ 61,776 ​ ​ — ​ ​ 103,433 ​ ​ 103,433 ​ ​ 57,848 ​ 2010 ​ LA2 ​ Los Angeles, CA ​ ​ 18,672 ​ ​ 94,114 ​ ​ 177,460 ​ ​ 18,672 ​ ​ 271,574 ​ ​ 290,246 ​ ​ 94,256 ​ 2010 ​ LA3 ​ Los Angeles, CA ​ ​ 9,795 ​ ​ — ​ ​ 48,986 ​ ​ — ​ ​ 58,781 ​ ​ 58,781 ​ ​ — ​ 2010 ​ LA4 ​ Los Angeles, CA ​ ​ — ​ ​ 2,533 ​ ​ 382 ​ ​ — ​ ​ 2,915 ​ ​ 2,915 ​ ​ 890 ​ 2018 ​ MI1 ​ Miami, FL ​ ​ 728 ​ ​ 9,325 ​ ​ 5,299 ​ ​ 728 ​ ​ 14,624 ​ ​ 15,352 ​ ​ 4,938 ​ 2010 ​ NY1 ​ New York, NY ​ ​ — ​ ​ — ​ ​ 43,340 ​ ​ — ​ ​ 43,340 ​ ​ 43,340 ​ ​ 29,633 ​ 2007 ​ NY2 ​ Secaucus, NJ ​ ​ 4,952 ​ ​ 18,408 ​ ​ 148,046 ​ ​ 2,729 ​ ​ 168,677 ​ ​ 171,406 ​ ​ 37,441 ​ 2013 ​ SV1 ​ San Jose, CA ​ ​ 6,863 ​ ​ 91,719 ​ ​ 44,091 ​ ​ 6,863 ​ ​ 135,810 ​ ​ 142,673 ​ ​ 49,727 ​ 2010 ​ SV2 ​ Milpitas, CA ​ ​ 5,086 ​ ​ 5,046 ​ ​ 28,342 ​ ​ 5,086 ​ ​ 33,388 ​ ​ 38,474 ​ ​ 23,578 ​ 2006 ​ SV3 ​ Santa Clara, CA ​ ​ 4,162 ​ ​ 3,482 ​ ​ 50,985 ​ ​ 3,972 ​ ​ 54,657 ​ ​ 58,629 ​ ​ 44,812 ​ 2007 ​ SV4 ​ Santa Clara, CA ​ ​ 4,632 ​ ​ 3,716 ​ ​ 99,272 ​ ​ 4,501 ​ ​ 103,119 ​ ​ 107,620 ​ ​ 53,801 ​ 2007 ​ SV5 ​ Santa Clara, CA ​ ​ 2,572 ​ ​ — ​ ​ 23,577 ​ ​ 2,544 ​ ​ 23,605 ​ ​ 26,149 ​ ​ 3,844 ​ 2007 ​ SV6 ​ Santa Clara, CA ​ ​ 4,741 ​ ​ — ​ ​ 36,758 ​ ​ 4,825 ​ ​ 36,674 ​ ​ 41,499 ​ ​ 3,377 ​ 2007 ​ SV7 ​ Santa Clara, CA ​ ​ 3,793 ​ ​ — ​ ​ 246,559 ​ ​ 3,595 ​ ​ 246,757 ​ ​ 250,352 ​ ​ 44,557 ​ 2007 ​ SV8 ​ Santa Clara, CA ​ ​ 12,158 ​ ​ — ​ ​ 184,198 ​ ​ 8,575 ​ ​ 187,781 ​ ​ 196,356 ​ ​ 1,989 ​ 2017 ​ SV9 ​ Santa Clara, CA ​ ​ 25,767 ​ ​ 407 ​ ​ 1,440 ​ ​ — ​ ​ 27,614 ​ ​ 27,614 ​ ​ 321 ​ 2019 ​ VA1 ​ Reston, VA ​ ​ 6,903 ​ ​ 32,939 ​ ​ 95,345 ​ ​ 6,779 ​ ​ 128,408 ​ ​ 135,187 ​ ​ 81,567 ​ 2007 ​ VA2 ​ Reston, VA ​ ​ 5,197 ​ ​ — ​ ​ 135,290 ​ ​ 5,539 ​ ​ 134,948 ​ ​ 140,487 ​ ​ 41,563 ​ 2007 ​ VA3 ​ Reston, VA ​ ​ 8,529 ​ ​ 26,411 ​ ​ 155,722 ​ ​ 3,866 ​ ​ 186,796 ​ ​ 190,662 ​ ​ 5,773 ​ 2016 ​ Reston Campus Expansion ​ Reston, VA ​ ​ 5,671 ​ ​ 17,538 ​ ​ 4 ​ ​ 5,672 ​ ​ 17,541 ​ ​ 23,213 ​ ​ 1,479 ​ 2016 ​ ​ ​ Total ​ $ 146,197 ​ $ 428,403 ​ $ 1,904,198 ​ $ 94,593 ​ $ 2,384,205 ​ $ 2,478,798 ​ $ 720,498 ​ ​ ​ ​ None of our properties are encumbered. The aggregate cost of the total properties for federal income tax purposes was approximately $2,133.9 million (unaudited) at December 31, 2019. See accompanying report of independent registered public accounting firm. ​ CoreSite Realty Corporation Schedule III Real Estate and Accumulated Depreciation (Continued) December 31, 2019 (in thousands) ​ The following table reconciles the historical cost and accumulated depreciation of our properties for the years ended December 31, 2019, 2018, and 2017: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2019 2018 2017 Property ​ ​ ​ ​ ​ ​ ​ ​ Balance, beginning of year ​ $ 2,083,205 ​ $ 1,821,217 $ 1,643,576 ​ Additions - property acquisitions ​ ​ 26,174 ​ ​ 7,474 ​ ​ 12,158 Additions - improvements ​ ​ 369,419 ​ ​ 254,514 ​ ​ 165,483 ​ Balance, end of year ​ $ 2,478,798 ​ $ 2,083,205 ​ $ 1,821,217 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated Depreciation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance, beginning of year ​ $ 590,784 ​ $ 473,141 ​ $ 369,303 ​ Additions, net of disposals ​ ​ 129,714 ​ ​ 117,643 ​ ​ 103,838 ​ Balance, end of year ​ $ 720,498 ​ $ 590,784 ​ $ 473,141 ​ ​ 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, 2019
Summary of Significant Accounting Policies
Principles of Consolidation and Basis of Presentation	Principles of Consolidation and Basis of Presentation The accompanying consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Fair Value Measurement ​ In August 2018, the FASB issued guidance codified in ASU 2018-13, Fair Value Measurement 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 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.
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 $13.6 million, $5.6 million and $3.3 million for the years ended December 31, 2019, 2018, and 2017, 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 ​
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 rental services, 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 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, 2019, and 2018, 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, 2019, 2018, or 2017.
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 $14.2 million, $15.6 million, and $16.3 million for the years ended December 31, 2019, 2018, and 2017, 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, 2019, and 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2019 2018 Internal sales commissions ​ $ 15,064 ​ $ 14,199 ​ Third party commissions ​ ​ 10,845 ​ ​ 9,855 ​ Other ​ ​ 462 ​ ​ 597 ​ Total ​ $ 26,371 ​ $ 24,651 ​
Deferred Financing Costs	Deferred Financing Costs ​
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, 2019, 2018, and 2017, no
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, 2019, 2018, and 2017, no
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 fixed power arrangements, a customer pays us a fixed monthly fee for a committed available amount of power. We recognize the fixed power revenue each month over the term of the lease. 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. ​ Allowance for Doubtful Accounts ​ 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, 2019, and 2018, the allowance for doubtful accounts totaled $0.4 million on the consolidated balance sheets at each date.
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 right-of-use (“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, 2019, and 2018, the amount included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.7 million and $1.6 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 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, 2019, and 2018, the gross deferred income taxes were not material. We currently have no
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, 2019
Summary of Significant Accounting Policies
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	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2019 2018 Internal sales commissions ​ $ 15,064 ​ $ 14,199 ​ Third party commissions ​ ​ 10,845 ​ ​ 9,855 ​ Other ​ ​ 462 ​ ​ 597 ​ Total ​ $ 26,371 ​ $ 24,651 ​

Investment in Real Estate (Tabl

Investment in Real Estate (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Buildings and ​ Construction in ​ ​ ​ ​ Market Land Improvements Progress Total Cost Boston ​ $ 5,154 ​ $ 119,227 ​ $ 931 ​ $ 125,312 ​ Chicago (1) ​ ​ 5,493 ​ ​ 115,699 ​ ​ 100,118 ​ ​ 221,310 ​ Denver ​ ​ — ​ ​ 32,659 ​ ​ 2,461 ​ ​ 35,120 ​ Los Angeles (1) ​ ​ 18,672 ​ ​ 376,525 ​ ​ 60,178 ​ ​ 455,375 ​ Miami ​ ​ 728 ​ ​ 14,491 ​ ​ 133 ​ ​ 15,352 ​ New York ​ ​ 2,729 ​ ​ 155,746 ​ ​ 56,271 ​ ​ 214,746 ​ Northern Virginia ​ ​ 21,856 ​ ​ 398,742 ​ ​ 101,619 ​ ​ 522,217 ​ San Francisco Bay (1) ​ ​ 39,961 ​ ​ 776,642 ​ ​ 72,763 ​ ​ 889,366 ​ Total ​ $ 94,593 ​ $ 1,989,731 ​ $ 394,474 ​ $ 2,478,798 ​ ​ (1) Refer to Note 3, Acquisitions, for further detail on CH2, U.S. Colo, SV8, and SV9 acquisitions. ​ The following is a summary of the properties owned or leased by market at December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Market Land Buildings and Construction in Total Cost ​ Boston ​ $ 5,154 ​ $ 107,596 ​ $ 1,644 ​ $ 114,394 ​ Chicago ​ ​ 5,493 ​ ​ 113,875 ​ ​ 14,478 ​ ​ 133,846 ​ Denver ​ ​ — ​ ​ 30,740 ​ ​ 476 ​ ​ 31,216 ​ Los Angeles ​ ​ 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 ​

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019, and 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2019 2018 Deferred rent receivable ​ $ 38,335 ​ $ 43,162 ​ Initial direct costs ​ ​ 26,371 ​ ​ 24,651 ​ Internal-use software ​ ​ 16,747 ​ ​ 17,708 ​ Prepaid expenses ​ ​ 7,675 ​ ​ 7,090 ​ Corporate furniture, fixtures and equipment ​ ​ 4,848 ​ ​ 5,006 ​ Deferred financing costs - revolving credit facility ​ ​ 3,148 ​ ​ 2,880 ​ Other ​ ​ 3,958 ​ ​ 1,793 ​ Total ​ $ 101,082 ​ $ 102,290 ​ ​
Estimated amortization of deferred leasing costs for each of the five succeeding fiscal years	​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2020 ​ $ 10,926 ​ 2021 ​ ​ 6,722 ​ 2022 ​ ​ 3,337 ​ 2023 ​ ​ 1,506 ​ 2024 ​ ​ 1,247 ​ Thereafter ​ ​ 2,633 ​ Total ​ $ 26,371 ​

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2019
Leases
Schedule of components of lease expense	The components of lease expense were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ ​ 2019 ​ 2018 ​ 2017 ​ Lease expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating lease expense ​ $ 26,566 ​ $ 22,927 ​ $ 20,345 ​ Variable lease expense ​ ​ 6,058 ​ ​ 4,769 ​ ​ 3,780 ​ Rent expense ​ $ 32,624 ​ $ 27,696 ​ $ 24,125 ​
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, 2019, are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ Operating ​ Year Ending December 31, ​ Leases ​ 2020 ​ $ 26,842 ​ 2021 ​ ​ 26,590 ​ 2022 ​ ​ 26,197 ​ 2023 ​ ​ 24,350 ​ 2024 ​ ​ 23,382 ​ Thereafter ​ ​ 106,361 ​ Total lease payments ​ $ 233,722 ​ Less imputed interest ​ ​ (46,279) ​ Total ​ $ 187,443 ​

Lease Revenue (Tables)

Lease Revenue (Tables)	12 Months Ended
	Dec. 31, 2019
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, ​ ​ ​ ​ 2019 ​ 2018 ​ 2017 ​ Lease revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum lease revenue ​ ​ $ 409,622 ​ $ 390,691 ​ $ 350,497 ​ Variable lease revenue ​ ​ ​ 87,354 ​ ​ 83,992 ​ ​ 69,031 ​ Total lease revenue ​ ​ $ 496,976 ​ $ 474,683 ​ $ 419,528 ​
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, 2019, are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum ​ Year Ending December 31, Lease Payments ​ 2020 ​ ​ $ 354,857 ​ 2021 ​ ​ ​ 254,291 ​ 2022 ​ ​ ​ 170,979 ​ 2023 ​ ​ ​ 111,055 ​ 2024 ​ ​ ​ 82,759 ​ Thereafter ​ ​ ​ 212,821 ​ Total ​ ​ $ 1,186,762 ​

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2019
Debt
Summary of outstanding indebtedness	A summary of outstanding indebtedness as of December 31, 2019, and 2018, is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maturity ​ December 31, ​ December 31, ​ ​ Interest Rate Date 2019 2018 Revolving credit facility ​ 3.01% and 3.95% at December 31, 2019, and December 31, 2018, respectively ​ November 8, 2023 ​ $ 62,500 ​ $ 211,500 ​ 2020 Senior unsecured term loan ​ 3.37% at December 31, 2018 ​ — ​ ​ — ​ ​ 150,000 ​ 2021 Senior unsecured term loan ​ 3.90% at December 31, 2018 ​ — ​ ​ — ​ ​ 100,000 ​ 2022 Senior unsecured term loan ​ 2.96% and 3.65% at December 31, 2019, and December 31, 2018, respectively ​ April 19, 2022 ​ ​ 200,000 ​ ​ 200,000 ​ 2023 Senior unsecured notes ​ 4.19% at December 31, 2019, and December 31, 2018, respectively ​ June 15, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured term loan (1) ​ 3.44% and 4.01% at December 31, 2019, and December 31, 2018, respectively ​ April 19, 2024 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured notes ​ 3.91% at December 31, 2019, and December 31, 2018, respectively ​ April 20, 2024 ​ ​ 175,000 ​ ​ 175,000 ​ 2025 Senior unsecured term loan (1) ​ 2.81% at December 31, 2019 ​ April 1, 2025 ​ ​ 350,000 ​ ​ — ​ 2026 Senior unsecured notes (1) ​ 4.52% at December 31, 2019 ​ April 17, 2026 ​ ​ 200,000 ​ ​ — ​ 2029 Senior unsecured notes ​ 4.31% at December 31, 2019 ​ April 17, 2029 ​ ​ 200,000 ​ ​ — ​ Total principal outstanding ​ ​ ​ ` ​ ​ 1,487,500 ​ ​ 1,136,500 ​ Unamortized deferred financing costs ​ ​ ​ ​ ​ ​ (9,098) ​ ​ (5,677) ​ Total debt ​ ​ ​ ​ ​ $ 1,478,402 ​ $ 1,130,823 ​ (1) Our Operating Partnership has in place swap agreements with respect to the term loans noted above. The interest rates presented represent the effective interest rates as of December 31, 2019, including the impact of the interest rate swaps, which effectively fix the interest rate on a portion of our variable rate debt. 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, ​ ​ 2020 ​ $ — ​ 2021 ​ ​ — ​ 2022 ​ ​ 200,000 ​ 2023 ​ ​ 212,500 ​ 2024 ​ ​ 325,000 ​ Thereafter ​ ​ 750,000 ​ Total principal outstanding ​ ​ 1,487,500 ​ Unamortized deferred financing costs ​ ​ (9,098) ​ Total debt, net ​ $ 1,478,402 ​

Derivatives and Hedging Activ_2

Derivatives and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2019
Derivatives and Hedging Activities
Schedule of derivative instruments	The following table summarizes our derivative positions as of the years ended December 31, 2019, and 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Notional Amount ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value (Level 2) (1) ​ December 31, ​ December 31, ​ Type of ​ Strike Rate ​ ​ Effective ​ Expiration ​ December 31, ​ December 31, ​ 2019 ​ 2018 ​ Derivative ​ as of 12/31/19 ​ ​ Date ​ Date ​ 2019 ​ 2018 ​ $ — ​ $ 50,000 ​ Interest Rate Swap ​ — % ​ 2/28/2014 ​ 1/31/2019 ​ $ — ​ $ 43 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 2.63 ​ ​ 5/5/2015 ​ 5/5/2020 ​ ​ 67 ​ ​ 1,099 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 3.92 ​ ​ 5/5/2018 ​ 4/5/2023 ​ ​ (2,819) ​ ​ (897) ​ ​ — ​ ​ 175,000 ​ Interest Rate Swap ​ — ​ ​ 6/3/2019 ​ 4/3/2019 ​ ​ — ​ ​ (3,183) ​ ​ 100,000 ​ ​ — ​ Interest Rate Swap ​ 2.79 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ 55 ​ ​ — ​ ​ 75,000 ​ ​ — ​ Interest Rate Swap ​ 2.79 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ 41 ​ ​ — ​ $ 325,000 ​ $ 375,000 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ (2,656) ​ $ (2,938) ​ (1) Derivative assets are recorded at fair value in our condensed consolidated balance sheets in other assets and derivative liabilities are recorded at fair value in our condensed consolidated balance sheets in unearned revenue, prepaid rent and other liabilities. We do not net our derivative position by counterparty for purposes of balance sheet presentation and disclosure.
Schedule of derivatives effect on consolidated financial staements	The following table shows the effect of the derivatives on our consolidated financial statements (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2019 ​ 2018 ​ 2017 ​ Cash Flow Hedges ​ ​ ​ ​ ​ ​ ​ ​ ​ Unrealized gain (loss) recognized in other comprehensive income $ (5,038) ​ $ (3,676) ​ $ 580 ​ Gain (loss) reclassified from other comprehensive income to interest expense ​ 212 ​ ​ (326) ​ ​ 623 ​ Interest expense ​ 41,712 ​ ​ 35,526 ​ ​ 24,147 ​

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2019
Stockholders' Equity
Schedule of dividends paid per share on Series A Cumulative Preferred Stock and common shares	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ Record Date ​ 2019 ​ 2018 ​ 2017 ​ Common Stock: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Ordinary income (1) ​ $ 3.07 ​ $ 3.09 ​ $ 2.13 ​ Qualified dividend ​ ​ — ​ ​ — ​ ​ — ​ Capital gains ​ ​ — ​ ​ — ​ ​ — ​ Return of capital ​ ​ 1.57 ​ ​ 0.93 ​ ​ 0.91 ​ Total dividend (2) ​ $ 4.64 ​ $ 4.02 ​ $ 3.04 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Preferred Stock: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Ordinary income ​ $ — ​ $ — ​ $ 1.36 ​ Qualified dividend ​ ​ — ​ ​ — ​ ​ — ​ Capital gains ​ ​ — ​ ​ — ​ ​ — ​ Total dividend (3) ​ $ — ​ $ — ​ $ 1.36 ​ (1) All 2018 and 2019 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 $1.22 per share or unit distribution was paid in January, all of which will apply to tax year 2020. A $1.10 per share or unit distribution was paid in January 2019, 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 applied 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, 2019
Noncontrolling Interests - Operating Partnership
Schedule of ownership interests in our Operating Partnership	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ December 31, 2018 ​ ​ Number of Units Percentage of Total ​ Number of Units Percentage of Total ​ CoreSite Realty Corporation ​ 37,244,987 ​ 77.6 % 36,256,032 ​ 75.8 % Noncontrolling interests ​ 10,771,658 ​ 22.4 ​ 11,575,268 ​ 24.2 ​ Total ​ 48,016,645 ​ 100.0 % 47,831,300 ​ 100.0 %

Equity Incentive Award Plan (Ta

Equity Incentive Award Plan (Tables)	12 Months Ended
	Dec. 31, 2019
Equity Incentive Award Plan
Schedule of stock option activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted ​ Weighted ​ Aggregate 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 ​ Granted ​ — ​ ​ — ​ ​ ​ ​ ​ Exercised ​ (17,387) ​ ​ 20.94 ​ ​ ​ $ 1.7 million ​ Forfeited ​ — ​ ​ — ​ ​ ​ ​ ​ Expired ​ — ​ ​ — ​ ​ ​ ​ ​ Options outstanding, December 31, 2019 ​ 31,746 ​ $ 19.57 ​ 1.4 years ​ $ 2.9 million ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of restricted awards and RSUs	​ ​ ​ ​ ​ ​ ​ ​ ​ Restricted Stock Weighted 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 ​ Granted ​ 160,361 ​ ​ 99.91 ​ Forfeited ​ (21,910) ​ ​ 94.03 ​ Vested ​ (130,957) ​ ​ 78.32 ​ Unvested balance, December 31, 2019 ​ 292,373 ​ $ 96.44 ​
Schedule of unvested PSAs and their weighted-average fair value at the date of grant	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ ​ ​ ​ Average Fair ​ ​ ​ Performance-Based Restricted Stock Awards ​ Value at Grant ​ ​ Minimum ​ Maximum Target Date Unvested balance, December 31, 2018 ​ 50,823 ​ 173,944 ​ 112,385 ​ $ 96.58 ​ Granted ​ 10,926 ​ 76,480 ​ 43,703 ​ ​ 114.55 ​ Performance adjustment (1) ​ 18,063 ​ (29,819) ​ (5,878) ​ ​ — ​ Forfeited ​ (1,538) ​ (5,604) ​ (3,571) ​ ​ 103.26 ​ Vested ​ (43,650) ​ (43,650) ​ (43,650) ​ ​ 84.78 ​ Unvested balance, December 31, 2019 ​ 34,624 ​ 171,351 ​ 102,989 ​ $ 107.84 ​ (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	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2019 2018 2017 Expected term (in years) ​ 2.82 ​ ​ 2.82 ​ 2.81 ​ Expected volatility ​ 24.09 % ​ 24.15 % 23.33 % Expected annual dividend (1) ​ — ​ ​ — ​ — ​ Risk-free rate ​ 2.48 % ​ 2.24 % 1.60 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2019
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, ​ ​ 2019 2018 2017 Net income attributable to common shares ​ $ 75,840 ​ $ 77,922 ​ $ 62,605 ​ Weighted-average common shares outstanding - basic ​ ​ 36,766,310 ​ ​ 34,957,244 ​ ​ 33,792,759 ​ Effect of potentially dilutive common shares: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options ​ ​ 36,685 ​ ​ 47,338 ​ ​ 88,399 ​ Unvested awards ​ ​ 140,878 ​ ​ 132,913 ​ ​ 177,791 ​ Weighted-average common shares outstanding - diluted ​ ​ 36,943,873 ​ ​ 35,137,495 ​ ​ 34,058,949 ​ Net income per share attributable to common shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ $ 2.06 ​ $ 2.23 ​ $ 1.85 ​ Diluted ​ $ 2.05 ​ $ 2.22 ​ $ 1.84 ​

Quarterly Financial Informati_2

Quarterly Financial Information (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
Quarterly Financial Information (unaudited)
Schedule of selected quarterly information	The table below reflects the selected quarterly information for the years ended December 31, 2019, and 2018 (in thousands except share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ ​ December 31, ​ September 30, ​ June 30, ​ March 31, ​ ​ 2019 2019 2019 2019 Revenue ​ $ 146,035 ​ $ 144,891 ​ $ 142,906 ​ $ 138,895 ​ Operating income ​ ​ 35,684 ​ ​ 33,643 ​ ​ 36,056 ​ ​ 35,433 ​ Net income ​ ​ 24,745 ​ ​ 22,644 ​ ​ 25,743 ​ ​ 25,905 ​ Net income attributable to common shares ​ ​ 19,194 ​ ​ 17,450 ​ ​ 19,535 ​ ​ 19,661 ​ Net income per share attributable to common shares - basic ​ $ 0.51 ​ $ 0.47 ​ $ 0.54 ​ $ 0.54 ​ Net income per share attributable to common shares - diluted ​ $ 0.51 ​ $ 0.47 ​ $ 0.53 ​ $ 0.54 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​

Organization and Description _2

Organization and Description of Business (Details) - Operating partnership	Dec. 31, 2019	Dec. 31, 2018
Coresite Realty Corporation
Organization
Percentage of ownership interests held by the entity	77.60%	75.80%
Noncontrolling interests
Organization
Percentage of ownership interests held by others	22.40%	24.20%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Real Estate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments in Real Estate
Capitalized interest costs	$ 13,600	$ 5,600	$ 3,300
Depreciation expense	$ 136,000	$ 122,900	$ 108,100
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_5

Summary of Significant Accounting Policies - Deferred Costs & Impairment (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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
Deferred Costs
Amortization of initial direct costs	14,200,000	15,600,000	16,300,000
Internal sales commissions	15,064,000	14,199,000
Third party commissions	10,845,000	9,855,000
Other	462,000	597,000
Total	26,371,000	24,651,000
Recoverability of Long-Lived Assets
Impairment of long-lived assets	$ 0	$ 0	$ 0

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Internal Use Software, RR & IT (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Internal-Use Software
Recognized impairment of internal use software	$ 0	$ 0	$ 0
Operating revenues:
Allowance for doubtful accounts (in dollars)	371	417
Asset Retirement and Environmental Remediation Obligations
Asset retirement and environmental remediation obligations	$ 1,700	$ 1,600
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_7

Summary of Significant Accounting Policies - Segment Information (Details)	12 Months Ended
	Dec. 31, 2019item
Segment information
Number of reportable segments	1

Acquisitions (Details)

Acquisitions (Details) $ in Millions	Apr. 12, 2019USD ($)ft²a	Apr. 20, 2018USD ($)ft²	Jan. 29, 2018USD ($)ft²a	Aug. 29, 2017USD ($)ft²a
Acquisitions
Net rentable space	200,000
SV8 Santa Clara CA
Acquisitions
Purchase price | $				$ 12.2
Area of land acquired | a				2
Net rentable space				162,000
Net Leased Space				108,000
SV9 Santa Clara CA
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			169,000
US Colo Los Angeles California
Acquisitions
Purchase price | $		$ 6.3
L A1 Facility Los Angeles CA
Acquisitions
Net Leased Space		6,723
LA4 Los Angeles CA
Acquisitions
Net Leased Space		21,850

Investment in Real Estate (Deta

Investment in Real Estate (Details) $ in Thousands	Dec. 31, 2019USD ($)	Apr. 12, 2019ft²	Dec. 31, 2018USD ($)
Investment in Real Estate
Land	$ 94,593		$ 86,955
Buildings and Improvements	1,989,731		1,730,329
Construction in Progress	394,474		265,921
Total investments in real estate	2,478,798		2,083,205
Net rentable space (in square feet) | ft²		200,000
Boston
Investment in Real Estate
Land	5,154		5,154
Buildings and Improvements	119,227		107,596
Construction in Progress	931		1,644
Total investments in real estate	125,312		114,394
Chicago
Investment in Real Estate
Land	5,493		5,493
Buildings and Improvements	115,699		113,875
Construction in Progress	100,118		14,478
Total investments in real estate	221,310		133,846
Denver
Investment in Real Estate
Buildings and Improvements	32,659		30,740
Construction in Progress	2,461		476
Total investments in real estate	35,120		31,216
Los Angeles
Investment in Real Estate
Land	18,672		18,672
Buildings and Improvements	376,525		338,011
Construction in Progress	60,178		51,688
Total investments in real estate	455,375		408,371
Miami
Investment in Real Estate
Land	728		728
Buildings and Improvements	14,491		14,014
Construction in Progress	133		69
Total investments in real estate	15,352		14,811
New York
Investment in Real Estate
Land	2,729		2,729
Buildings and Improvements	155,746		152,956
Construction in Progress	56,271		33,796
Total investments in real estate	214,746		189,481
Northern Virginia
Investment in Real Estate
Land	21,856		22,793
Buildings and Improvements	398,742		346,209
Construction in Progress	101,619		94,623
Total investments in real estate	522,217		463,625
San Francisco Bay
Investment in Real Estate
Land	39,961		31,386
Buildings and Improvements	776,642		626,928
Construction in Progress	72,763		69,147
Total investments in real estate	$ 889,366		$ 727,461

Other Assets (Details)

Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred rent receivable	$ 38,335	$ 43,162
Initial direct costs	26,371	24,651
Internal-use software	16,747	17,708
Prepaid expense	7,675	7,090
Corporate furniture, fixtures and equipment	4,848	5,006
Deferred financing costs - revolving credit facility	9,098	5,677
Other	3,958	1,793
Total	101,082	102,290
Estimated amortization of deferred leasing costs
2020	10,926
2021	6,722
2022	3,337
2023	1,506
2024	1,247
Thereafter	2,633
Total	26,371
Revolving Credit Facility
Deferred financing costs - revolving credit facility	$ 3,148	$ 2,880

Leases - (Detail)

Leases - (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Lease terms
Option to extend	P20Y
Weighted-average remaining lease term	9 years	10 years
Weighted average discount rate - Operating leases	4.90%
Increase in Right-of-Use Asset	$ 1,040	$ 114,989	$ 8,330
Lease expense
Operating lease expense	26,566	22,927	20,345
Variable lease expense	6,058	4,769	3,780
Rent expense	$ 32,624	$ 27,696	$ 24,125
Minimum
Lease terms
Remaining lease terms	1 year
Maximum
Lease terms
Remaining lease terms	10 years
DE1 Denver CO
Lease terms
Net Leased Space | ft²	5,300
Increase in Right-of-Use Asset	$ 1,000
Increase in operating lease liability	$ 1,000

Leases - Future Minimum Payment

Leases - Future Minimum Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2020	$ 26,842
2021	26,590
2022	26,197
2023	24,350
2024	23,382
Thereafter	106,361
Total lease payments	233,722
Less imputed interest	(46,279)
Total	$ 172,976	$ 190,304

Lease Revenue (Details)

Lease Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of lease revenue
Minimum lease revenue	$ 409,622	$ 390,691	$ 350,497
Variable lease revenue	87,354	83,992	69,031
Total lease revenue	496,976	$ 474,683	$ 419,528
Future minimum lease payments to be received under noncancelable leases
2020	354,857
2021	254,291
2022	170,979
2023	111,055
2024	82,759
Thereafter	212,821
Total	1,186,762
Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year	$ 26,842

Debt - Summary of Debt (Details

Debt - Summary of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Nov. 08, 2019	Apr. 17, 2019	Dec. 31, 2018
Debt
Total principal outstanding	$ 1,487,500			$ 1,136,500
Unamortized deferred financing costs	(9,098)			(5,677)
Total debt	$ 1,478,402			$ 1,130,823
Revolving Credit Facility
Debt
Interest rate as of reporting date (as a percent)	3.01%			3.95%
Total principal outstanding	$ 62,500			$ 211,500
Unamortized deferred financing costs	$ (3,148)			$ (2,880)
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)				3.37%
Total principal outstanding				$ 150,000
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)				3.90%
Total principal outstanding				$ 100,000
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	2.96%			3.65%
Total principal outstanding	$ 200,000			$ 200,000
2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	3.44%			4.01%
Total principal outstanding	$ 150,000			$ 150,000
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)	2.81%
Total principal outstanding	$ 350,000
2023 Senior unsecured notes | 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 notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)	3.91%			3.91%
Total principal outstanding	$ 175,000			$ 175,000
2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)	4.52%
Total principal outstanding	$ 200,000
2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)	4.31%
Total principal outstanding	$ 200,000
Operating partnership | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		$ 150,000
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		100,000
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	$ 200,000
Operating partnership | 2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		150,000
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		$ 350,000
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)	4.52%		4.11%

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility (Details) $ in Thousands	Nov. 08, 2019USD ($)item	Apr. 19, 2018	Dec. 31, 2019USD ($)	Nov. 07, 2019USD ($)	Dec. 31, 2018USD ($)	Apr. 19, 2017USD ($)
Debt
Total principal outstanding			$ 1,487,500		$ 1,136,500
Revolving Credit Facility
Debt
Total principal outstanding			62,500		211,500
Credit Agreement
Debt
Letters of credit outstanding			$ 4,900
Leverage ratio (as a percent)			29.00%
Secured debt ratio, Covenant (as a percent)			0.00%
Fixed charge coverage ratio			5.9
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%
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding					150,000
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding					100,000
2022 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)			29.00%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 200,000		200,000
2023 Senior unsecured notes
Debt
Leverage ratio (as a percent)			29.00%
2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 150,000		$ 150,000
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			350,000
Operating partnership | Revolving Credit Facility
Debt
Maximum borrowing capacity	$ 450,000
Operating partnership | Credit Agreement
Debt
Number of options available for extensions | item	1
Extension fee (as a percent)	10.00%
Total increase in commitment under accordion feature	$ 550,000
Additional liquidity	100,000
Maximum borrowing capacity	950,000,000			$ 850,000
Increase in accordion feature borrowing capacity	200,000
Maximum borrowing capacity including accordion feature	1,500,000
Borrowing capacity at end of period			450,000
Amount available for borrowing under the facility	950,000		$ 382,600
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 | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	150,000
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	$ 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
Total principal outstanding			$ 200,000
Unsecured term loan						$ 100,000
Increase in accordion feature borrowing capacity						$ 300,000
Operating partnership | 2023 Senior unsecured notes | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	0.40%
Operating partnership | 2023 Senior unsecured notes | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	1.00%
Operating partnership | 2024 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			1.40%
Operating partnership | 2024 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)		1.40%
Operating partnership | 2024 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)		2.00%
Operating partnership | 2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	$ 150,000
Unsecured term loan	150,000
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	350,000
Unsecured term loan	$ 350,000
Leverage ratio (as a percent)			29.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			120.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	120.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	180.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	20.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	80.00%

Debt - Unsecured Debt (Details)

Debt - Unsecured Debt (Details) - USD ($) $ in Thousands	Nov. 08, 2019	Apr. 19, 2018	Dec. 31, 2019	Jul. 17, 2019	Apr. 17, 2019	Dec. 31, 2018	Apr. 20, 2017	Apr. 19, 2017	Jun. 15, 2016
Debt
Total principal outstanding			$ 1,487,500			$ 1,136,500
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding						$ 150,000
Effective interest rate						3.37%
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding						$ 100,000
Effective interest rate						3.90%
2022 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)			29.00%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 200,000			$ 200,000
Effective interest rate			2.96%			3.65%
2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 150,000			$ 150,000
Effective interest rate			3.44%			4.01%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 350,000
Effective interest rate			2.81%
2023 Senior unsecured notes
Debt
Leverage ratio (as a percent)			29.00%
2023 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 150,000			$ 150,000
Effective interest rate			4.19%			4.19%
2024 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 175,000			$ 175,000
Effective interest rate			3.91%			3.91%
2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 200,000
Effective interest rate			4.52%
2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 200,000
Effective interest rate			4.31%
Operating partnership | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	$ 150,000
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	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
Total principal outstanding			$ 200,000
Unsecured term loan								$ 100,000
Increase in accordion feature borrowing capacity								$ 300,000
Operating partnership | 2024 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			1.40%
Operating partnership | 2024 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)		1.40%
Operating partnership | 2024 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)		2.00%
Operating partnership | 2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	150,000
Unsecured term loan	150,000
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding	350,000
Unsecured term loan	$ 350,000
Leverage ratio (as a percent)			29.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			120.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	120.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	180.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	20.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	80.00%
Operating partnership | 2023 Senior unsecured notes | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	0.40%
Operating partnership | 2023 Senior unsecured notes | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	1.00%
Operating partnership | 2023 Senior unsecured notes | 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 notes | 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%
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Effective interest rate			4.52%		4.11%
Unsecured term loan					$ 200,000
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 | 2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Unsecured term loan				$ 75,000
Aggregate principal amount					$ 125,000
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 | 2029 Senior unsecured notes | Senior Unsecured Notes | Private Placement
Debt
Unsecured term loan					$ 200,000
Stated interest rate					4.31%

Debt - Maturities (Details)

Debt - Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt maturities
2022	$ 200,000
2023	212,500
2024	325,000
Thereafter	750,000
Total principal outstanding	1,487,500	$ 1,136,500
Unamortized deferred financing costs	(9,098)	(5,677)
Total debt	$ 1,478,402	$ 1,130,823

Derivatives and Hedging Activ_3

Derivatives and Hedging Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative instruments and hedging activities disclosures
Unrealized (loss) gain recorded in other comprehensive income		$ (5,038)	$ (3,676)	$ 580
Interest expense		41,712	35,526	24,147
Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount		325,000	375,000
Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value		(2,656)	(2,938)
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Unrealized (loss) gain recorded in other comprehensive income		(5,038)	(3,676)	580
Amount reclassified to interest expense		212	(326)	623
Interest expense		$ 41,712	35,526	$ 24,147
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	$ 1,600
Interest rate swap expiring 1/31/2019 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			50,000
Interest rate swap expiring 1/31/2019 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			43
Interest rate swap expiring 5/20/2020
Derivative instruments and hedging activities disclosures
Strike Rate		2.63%
Interest rate swap expiring 5/20/2020 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount		$ 75,000	75,000
Interest rate swap expiring 5/20/2020 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value		$ 67	1,099
Interest rate swap expiring 4/5/2023
Derivative instruments and hedging activities disclosures
Strike Rate		3.92%
Interest rate swap expiring 4/5/2023 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount		$ 75,000	75,000
Interest rate swap expiring 4/5/2023 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value		$ (2,819)	(897)
Interest rate swap expiring 4/3/2019 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			175,000
Interest rate swap expiring 4/3/2019 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (3,183)
Interest rate swap expiring 4/1/2025
Derivative instruments and hedging activities disclosures
Strike Rate		2.79%
Interest rate swap expiring 4/1/2025 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount		$ 100,000
Interest rate swap expiring 4/1/2025 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value		$ 55
Second interest rate swap expring 4/12025
Derivative instruments and hedging activities disclosures
Strike Rate		2.79%
Second interest rate swap expring 4/12025 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount		$ 75,000
Second interest rate swap expring 4/12025 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value		$ 41

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	Dec. 12, 2017	Dec. 12, 2012	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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			37,701,042	36,708,691
Common Stock, shares outstanding			37,701,042	36,708,691
Proceeds from exercise of stock options			$ 364,000	$ 310,000	$ 4,820,000
Redemption of preferred stock					115,000,000
2010 Plan
Stockholders' Equity
Proceeds from exercise of stock options			400,000	300,000
Amount of shares of common stock issued under the plan			$ 188,741	$ 206,808	329,033
Payments made for exercise of stock options					$ 4,800,000
Operating partnership
Stockholders' Equity
Number of Operating Partnership Units (in shares)			48,016,645	47,831,300
Common Operating Partnership units redeemed for common stock (in shares)			3,610	11,068	15,011
Series A 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
Original issue costs	$ 4,300,000
Series A Cumulative Preferred Stock, dividend rate (as a percent)	7.25%	7.25%
Series A Cumulative Preferred Stock, liquidation preference (in dollars per share)		$ 25
Carlyle Group | Operating partnership
Stockholders' Equity
Common Operating Partnership units redeemed for common stock (in shares)			800,000	2,250,000	3,000,000

Stockholders' Equity - Dividend

Stockholders' Equity - Dividends (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Jan. 31, 2020	Jan. 31, 2019	Dec. 31, 2018	Jan. 31, 2018	Dec. 31, 2017	Jan. 31, 2017	Dec. 31, 2016
Dividends
Dividend deduction (as a percent)	20.00%
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)	$ 3.07			$ 3.09		$ 2.13
Return of capital	1.57			0.93		0.91
Total dividend (in dollars per share)	$ 4.64			$ 4.02		3.04		$ 2.36
Ordinary dividend paid (in dollars per share)			$ 1.10		$ 0.98	0.44	$ 0.80
Common Stock | Subsequent Event
Dividends
Ordinary dividend paid (in dollars per share)		$ 1.22
Preferred Stock
Dividends
Ordinary income (in dollars per share)						1.36
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, 2018$ / sharesshares	Sep. 28, 2018$ / shares	Jun. 29, 2018$ / shares	Mar. 29, 2018$ / shares	Dec. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares
Noncontrolling Interests - Operating Partnership
Shares of common stock issued related to employee compensation arrangements					185,345
Closing price of the Company's stock (in dollars per share) | $ / shares					$ 112.12
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 | $					$ 5,248	$ 21,002	$ 167
Noncontrolling Interests | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					803,610	2,261,068	15,011
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $					$ 5,200	$ 21,000	$ 200
Operating partnership
Noncontrolling Interests - Operating Partnership
Conversion ratio for common operating partnership units					1
Number of Operating Partnership Units (in shares)	47,831,300				48,016,645	47,831,300
Percentage of Total	100.00%				100.00%	100.00%
Quarterly distributions will receive by holders of operating partnership units (in dollars per unit) | $ / shares	$ 1.22	$ 1.22	$ 1.22	$ 1.10
Common Operating Partnership units redeemed for common stock (in shares)					3,610	11,068	15,011
Operating partnership | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					800,000	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				37,244,987	36,256,032
Percentage of Total	75.80%				77.60%	75.80%
Operating partnership | Noncontrolling interests
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)	11,575,268				10,771,658	11,575,268
Percentage of Total	24.20%				22.40%	24.20%
Operating partnership | Noncontrolling Interests
Noncontrolling Interests - Operating Partnership
Redemption value of the noncontrolling interest | $					$ 1,200,000

Equity Incentive Plan (Details)

Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity Incentive Plan
Shares of common stock reserved for issuance	6,000,000
Shares of common stock that were available for issuance	2,674,745
Stock Options
Number of Shares Subject to Option
Options outstanding at the beginning of the period (in shares)	49,133	67,789	265,550
Exercised (in shares)	(17,387)	(18,656)	(197,761)
Options outstanding at the end of the period (in shares)	31,746	49,133	67,789	265,550
Weighted Average Remaining Contractual Term
Options outstanding at the end of the period	1 year 4 months 24 days	2 years 7 months 6 days	3 years 4 months 24 days	5 years
Aggregate Intrinsic Value
Outstanding at the end of the period		$ 3.3	$ 6.4	$ 15
Exercised	$ 1.7	$ 1.6	$ 13.3
Exercisable at the end of the period	$ 2.9
Stock Options | Weighted Average
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 20.06	$ 19.12	$ 22.96
Exercised (in dollars per share)	20.94	16.63	24.37
Options outstanding at the end of the period (in dollars per share)	$ 19.57	$ 20.06	$ 19.12	$ 22.96
Restricted Awards
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	284,879	289,843	323,641
Granted (in shares)	160,361	155,697	139,788
Forfeited (in shares)	(21,910)	(36,546)	(34,020)
Vested (in shares)	(130,957)	(124,115)	(139,566)
Unvested balance at the end of the period (in shares)	292,373	284,879	289,843	323,641
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 85.98	$ 70.71	$ 53.84
Granted (in dollars per share)	99.91	96.47	85.79
Forfeited (in dollars per share)	94.03	84.10	64
Vested (in dollars per share)	78.32	64.03	44.31
Unvested balance at the end of the period (in dollars per share)	$ 96.44	85.98	$ 70.71	$ 53.84
Compensation cost not yet recognized
Unearned compensation	$ 19.1
Vesting period	2 years 4 months 24 days
PSAs
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 96.58
Granted (in dollars per share)	114.55
Forfeited (in dollars per share)	103.26
Vested (in dollars per share)	84.78
Unvested balance at the end of the period (in dollars per share)	$ 107.84	$ 96.58
Compensation cost not yet recognized
Unearned compensation	$ 5.6
Assumptions used to value the stock options granted
Expected term	2 years 9 months 25 days	2 years 9 months 25 days	2 years 9 months 21 days
Expected volatility (as a percent)	24.09%	24.15%	23.33%
Risk-free rate (as a percent)	2.48%	2.24%	1.60%
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)	50,823
Granted (in shares)	10,926
Performance adjustment	18,063
Forfeited (in shares)	(1,538)
Vested (in shares)	(43,650)
Unvested balance at the end of the period (in shares)	34,624	50,823
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)	173,944
Granted (in shares)	76,480
Performance adjustment	(29,819)
Forfeited (in shares)	(5,604)
Vested (in shares)	(43,650)
Unvested balance at the end of the period (in shares)	171,351	173,944
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)	112,385
Granted (in shares)	43,703
Performance adjustment	(5,878)
Forfeited (in shares)	(3,571)
Vested (in shares)	(43,650)
Unvested balance at the end of the period (in shares)	102,989	112,385

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share
Net income attributable to common shares	$ 19,194	$ 17,450	$ 19,535	$ 19,661	$ 19,631	$ 18,600	$ 19,389	$ 20,302	$ 75,840	$ 77,922	$ 62,605
Weighted-average common shares outstanding - basic									36,766,310	34,957,244	33,792,759
Effect of potentially dilutive common shares:
Weighted-average common shares outstanding - diluted									36,943,873	35,137,495	34,058,949
Net income per share attributable to common shares
Basic	$ 0.51	$ 0.47	$ 0.54	$ 0.54	$ 0.54	$ 0.52	$ 0.57	$ 0.60	$ 2.06	$ 2.23	$ 1.85
Diluted	$ 0.51	$ 0.47	$ 0.53	$ 0.54	$ 0.54	$ 0.52	$ 0.57	$ 0.59	$ 2.05	$ 2.22	$ 1.84
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share									2,546	132,749	24,961
Stock Options
Effect of potentially dilutive common shares:
Potentially dilutive common shares									36,685	47,338	88,399
Restricted awards
Effect of potentially dilutive common shares:
Potentially dilutive common shares									140,878	132,913	177,791

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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)	4.00%	3.00%
Company contributions under certain 401(k) savings plans		$ 1,700	$ 1,210	$ 1,100

Estimated Fair Value of Finan_2

Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Estimated Fair Value of Financial Instruments
Total principal outstanding	$ 1,487,500	$ 1,136,500

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Thousands	Dec. 20, 2019	Apr. 27, 2018	Dec. 31, 2020	Dec. 31, 2019
Telecommunications Capacity
Commitments and Contingencies
Open commitments				$ 74,300
Telecommunications Capacity | Forecast
Commitments and Contingencies
Open commitments met			$ 6,500
Construction Contracts
Commitments and Contingencies
Open commitments				$ 189,400
Pending Litigation
Commitments and Contingencies
Sought amount		$ 21,000
Pending Litigation | Damages Related To Construction Project
Commitments and Contingencies
Sought amount		$ 2,800
Settled Litigation
Commitments and Contingencies
Estimated reimbursement amount	$ 3,100
Settled Litigation | Prime Rate
Commitments and Contingencies
Interest rate margin (as a percent)	2.00%

Quarterly Financial Informati_3

Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information (unaudited)
Revenues	$ 146,035	$ 144,891	$ 142,906	$ 138,895	$ 139,146	$ 139,180	$ 136,447	$ 129,619	$ 572,727	$ 544,392	$ 481,821
Operating income	35,684	33,643	36,056	35,433	35,366	34,473	36,103	36,337	140,816	142,279	124,812
Net income	24,745	22,644	25,743	25,905	25,898	25,020	27,279	28,566	99,037	106,763	100,491
Net income attributable to common shares	$ 19,194	$ 17,450	$ 19,535	$ 19,661	$ 19,631	$ 18,600	$ 19,389	$ 20,302	$ 75,840	$ 77,922	$ 62,605
Net income per share attributable to common shares - basic (in dollars per share)	$ 0.51	$ 0.47	$ 0.54	$ 0.54	$ 0.54	$ 0.52	$ 0.57	$ 0.60	$ 2.06	$ 2.23	$ 1.85
Net income per share attributable to common shares - diluted (in dollars per share)	$ 0.51	$ 0.47	$ 0.53	$ 0.54	$ 0.54	$ 0.52	$ 0.57	$ 0.59	$ 2.05	$ 2.22	$ 1.84

Schedule III Real Estate and _2

Schedule III Real Estate and Accumulated Depreciation - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Initial Cost
Land	$ 146,197
Building and Improvements	428,403
Costs Capitalized Subsequent to Acquisition	1,904,198
Gross Amount Carried As of Year End
Land	94,593
Building and Improvements	2,384,205
Total	2,478,798	$ 2,083,205	$ 1,821,217	$ 1,643,576
Accumulated Depreciation	720,498	$ 590,784	$ 473,141	$ 369,303
Aggregate cost of the total properties for federal income tax purposes	2,133,900
BO1 Somerville MA
Initial Cost
Land	6,100
Building and Improvements	26,748
Costs Capitalized Subsequent to Acquisition	92,464
Gross Amount Carried As of Year End
Land	5,154
Building and Improvements	120,158
Total	125,312
Accumulated Depreciation	66,223
CH1 Chicago IL
Initial Cost
Land	5,493
Building and Improvements	49,522
Costs Capitalized Subsequent to Acquisition	66,321
Gross Amount Carried As of Year End
Land	5,493
Building and Improvements	115,843
Total	121,336
Accumulated Depreciation	52,821
CH2 Chicago IL
Initial Cost
Land	4,383
Costs Capitalized Subsequent to Acquisition	95,591
Gross Amount Carried As of Year End
Building and Improvements	99,974
Total	99,974
DC1 Washington DC
Initial Cost
Building and Improvements	4,797
Costs Capitalized Subsequent to Acquisition	5,311
Gross Amount Carried As of Year End
Building and Improvements	10,108
Total	10,108
Accumulated Depreciation	7,404
DC2 Washington DC
Initial Cost
Costs Capitalized Subsequent to Acquisition	22,560
Gross Amount Carried As of Year End
Building and Improvements	22,560
Total	22,560
Accumulated Depreciation	2,336
DE1 Denver CO
Initial Cost
Building and Improvements	37
Costs Capitalized Subsequent to Acquisition	32,647
Gross Amount Carried As of Year End
Building and Improvements	32,684
Total	32,684
Accumulated Depreciation	9,238
DE2 Denver CO
Initial Cost
Building and Improvements	4
Costs Capitalized Subsequent to Acquisition	2,432
Gross Amount Carried As of Year End
Building and Improvements	2,436
Total	2,436
Accumulated Depreciation	1,081
LA1 Los Angeles CA [Member]
Initial Cost
Building and Improvements	41,657
Costs Capitalized Subsequent to Acquisition	61,776
Gross Amount Carried As of Year End
Building and Improvements	103,433
Total	103,433
Accumulated Depreciation	57,848
LA2 Los Angeles CA [Member]
Initial Cost
Land	18,672
Building and Improvements	94,114
Costs Capitalized Subsequent to Acquisition	177,460
Gross Amount Carried As of Year End
Land	18,672
Building and Improvements	271,574
Total	290,246
Accumulated Depreciation	94,256
LA3 Los Angeles CA
Initial Cost
Land	9,795
Costs Capitalized Subsequent to Acquisition	48,986
Gross Amount Carried As of Year End
Building and Improvements	58,781
Total	58,781
LA4 Los Angeles CA
Initial Cost
Building and Improvements	2,533
Costs Capitalized Subsequent to Acquisition	382
Gross Amount Carried As of Year End
Building and Improvements	2,915
Total	2,915
Accumulated Depreciation	890
MI1 Miami FL [Member]
Initial Cost
Land	728
Building and Improvements	9,325
Costs Capitalized Subsequent to Acquisition	5,299
Gross Amount Carried As of Year End
Land	728
Building and Improvements	14,624
Total	15,352
Accumulated Depreciation	4,938
NY1 New York NY [Member]
Initial Cost
Costs Capitalized Subsequent to Acquisition	43,340
Gross Amount Carried As of Year End
Building and Improvements	43,340
Total	43,340
Accumulated Depreciation	29,633
NY2 Secaucus NJ [Member]
Initial Cost
Land	4,952
Building and Improvements	18,408
Costs Capitalized Subsequent to Acquisition	148,046
Gross Amount Carried As of Year End
Land	2,729
Building and Improvements	168,677
Total	171,406
Accumulated Depreciation	37,441
SV1 San Jose CA [Member]
Initial Cost
Land	6,863
Building and Improvements	91,719
Costs Capitalized Subsequent to Acquisition	44,091
Gross Amount Carried As of Year End
Land	6,863
Building and Improvements	135,810
Total	142,673
Accumulated Depreciation	49,727
SV2 Milpitas CA [Member]
Initial Cost
Land	5,086
Building and Improvements	5,046
Costs Capitalized Subsequent to Acquisition	28,342
Gross Amount Carried As of Year End
Land	5,086
Building and Improvements	33,388
Total	38,474
Accumulated Depreciation	23,578
SV3 Santa Clara CA [Member]
Initial Cost
Land	4,162
Building and Improvements	3,482
Costs Capitalized Subsequent to Acquisition	50,985
Gross Amount Carried As of Year End
Land	3,972
Building and Improvements	54,657
Total	58,629
Accumulated Depreciation	44,812
SV4 Santa Clara CA [Member]
Initial Cost
Land	4,632
Building and Improvements	3,716
Costs Capitalized Subsequent to Acquisition	99,272
Gross Amount Carried As of Year End
Land	4,501
Building and Improvements	103,119
Total	107,620
Accumulated Depreciation	53,801
SV5 Santa Clara CA [Member]
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	3,844
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	3,377
SV7 Santa Clara CA
Initial Cost
Land	3,793
Costs Capitalized Subsequent to Acquisition	246,559
Gross Amount Carried As of Year End
Land	3,595
Building and Improvements	246,757
Total	250,352
Accumulated Depreciation	44,557
SV8 Santa Clara CA
Initial Cost
Land	12,158
Costs Capitalized Subsequent to Acquisition	184,198
Gross Amount Carried As of Year End
Land	8,575
Building and Improvements	187,781
Total	196,356
Accumulated Depreciation	1,989
SV9 Santa Clara CA
Initial Cost
Land	25,767
Building and Improvements	407
Costs Capitalized Subsequent to Acquisition	1,440
Gross Amount Carried As of Year End
Building and Improvements	27,614
Total	27,614
Accumulated Depreciation	321
VA1 Reston VA [Member]
Initial Cost
Land	6,903
Building and Improvements	32,939
Costs Capitalized Subsequent to Acquisition	95,345
Gross Amount Carried As of Year End
Land	6,779
Building and Improvements	128,408
Total	135,187
Accumulated Depreciation	81,567
VA2 Reston VA [Member]
Initial Cost
Land	5,197
Costs Capitalized Subsequent to Acquisition	135,290
Gross Amount Carried As of Year End
Land	5,539
Building and Improvements	134,948
Total	140,487
Accumulated Depreciation	41,563
VA3 Reston VA
Initial Cost
Land	8,529
Building and Improvements	26,411
Costs Capitalized Subsequent to Acquisition	155,722
Gross Amount Carried As of Year End
Land	3,866
Building and Improvements	186,796
Total	190,662
Accumulated Depreciation	5,773
Reston Campus Expansion
Initial Cost
Land	5,671
Building and Improvements	17,538
Costs Capitalized Subsequent to Acquisition	4
Gross Amount Carried As of Year End
Land	5,672
Building and Improvements	17,541
Total	23,213
Accumulated Depreciation	$ 1,479

Schedule III Real Estate and _3

Schedule III Real Estate and Accumulated Depreciation - Rollforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property
Balance, beginning of year	$ 2,083,205	$ 1,821,217	$ 1,643,576
Additions - property acquisitions	26,174	7,474	12,158
Additions - improvements	369,419	254,514	165,483
Balance, end of year	2,478,798	2,083,205	1,821,217
Accumulated Depreciation
Balance, beginning of year	590,784	473,141	369,303
Additions, net of disposals	129,714	117,643	103,838
Balance, end of year	$ 720,498	$ 590,784	$ 473,141