CoreSite Realty 10-K 5 Feb 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 03, 2021	Jun. 30, 2020
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
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
ICFR Auditor Attestation Flag	true
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		42,767,634
Entity Public Float			$ 4,439.8
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
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, 2020	Dec. 31, 2019
Investments in real estate:
Land	$ 104,734	$ 94,593
Buildings and improvements	2,273,536	1,989,731
Total investments in real estate	2,378,270	2,084,324
Less: Accumulated depreciation and amortization	(867,975)	(720,498)
Net investment in operating properties	1,510,295	1,363,826
Construction in progress	319,411	394,474
Net investments in real estate	1,829,706	1,758,300
Operating lease right-of-use assets, net	173,928	172,976
Cash and cash equivalents	5,543	3,048
Accounts and other receivables, net of allowance for doubtful accounts of $511 and $371 as of December 31, 2020, and December 31, 2019, respectively	20,849	21,008
Lease intangibles, net of accumulated amortization of $3,913 and $4,022 as of December 31, 2020, and December 31, 2019, respectively	2,507	3,939
Goodwill	40,646	40,646
Other assets, net	103,094	101,082
Total assets	2,176,273	2,100,999
Liabilities:
Debt, net of unamortized deferred financing costs of $7,589 and $9,098 as of December 31, 2020, and December 31, 2019, respectively	1,715,911	1,478,402
Operating lease liabilities	189,404	187,443
Accounts payable and accrued expenses	79,140	123,304
Accrued dividends and distributions	63,878	62,332
Acquired below-market lease contracts, net of accumulated amortization of $1,684 and $1,511 as of December 31, 2020, and December 31, 2019, respectively	2,313	2,511
Unearned revenue, prepaid rent and other liabilities	53,149	33,119
Total liabilities	2,103,795	1,887,111
Stockholders' equity:
Common Stock, par value $0.01, 100,000,000 shares authorized and 42,768,153 and 37,701,042 shares issued and outstanding at December 31, 2020, and December 31, 2019, respectively	422	373
Additional paid-in capital	555,595	512,324
Accumulated other comprehensive loss	(20,526)	(6,026)
Distributions in excess of net income	(471,910)	(348,509)
Total stockholders' equity	63,581	158,162
Noncontrolling interests	8,897	55,726
Total equity	72,478	213,888
Total liabilities and equity	$ 2,176,273	$ 2,100,999

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
CONDENSED CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts (in dollars)	$ 511	$ 371
Accumulated amortization, lease intangibles (in dollars)	3,913	4,022
Deferred financing costs - revolving credit facility	7,589	9,098
Accumulated amortization of below-market lease	$ 1,684	$ 1,511
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	42,768,153	37,701,042
Common Stock, shares outstanding	42,768,153	37,701,042

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Data center revenue:
Interconnection revenue	$ 84,073	$ 75,751	$ 69,709
Total operating revenues	606,824	572,727	544,392
Operating expenses:
Property operating and maintenance	169,855	157,293	152,357
Real estate taxes and insurance	23,996	22,866	19,239
Depreciation and amortization	168,915	152,925	141,633
Sales and marketing	23,726	22,439	21,023
General and administrative	44,026	43,764	40,090
Rent	36,547	32,624	27,696
Transaction costs			75
Total operating expenses	467,065	431,911	402,113
Operating income	139,759	140,816	142,279
Interest expense	(45,086)	(41,712)	(35,526)
Income before income taxes	94,673	99,104	106,753
Income tax (expense) benefit	(56)	(67)	10
Net income	94,617	99,037	106,763
Net income attributable to noncontrolling interests	15,308	23,197	28,841
Net income attributable to common shares	$ 79,309	$ 75,840	$ 77,922
Net income per share attributable to common shares:
Basic	$ 1.96	$ 2.06	$ 2.23
Diluted	$ 1.95	$ 2.05	$ 2.22
Weighted average common shares outstanding
Basic	40,453,438	36,766,310	34,957,244
Diluted	40,634,324	36,943,873	35,137,495
Rental, power, and related revenue
Data center revenue:
Rental revenue	$ 512,343	$ 485,131	$ 463,086
Office, light-industrial and other revenue
Data center revenue:
Rental revenue	$ 10,408	$ 11,845	$ 11,597

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 94,617	$ 99,037	$ 106,763
Other comprehensive (loss) income:
Unrealized loss on derivative contracts	(20,647)	(5,038)	(3,676)
Reclassification of otehr comprehensive (income) loss to interest expense	5,016	212	(326)
Comprehensive income	78,986	94,211	102,761
Comprehensive income attributable to noncontrolling interests	10,988	22,030	27,912
Comprehensive income attributable to CoreSite Realty Corporation	$ 67,998	$ 72,181	$ 74,849

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF EQUITY - USD ($) $ in Thousands	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, 2017	$ 338	$ 457,495	$ 753	$ (177,566)	$ 281,020	$ 137,624	$ 418,644
Balance (in shares) at Dec. 31, 2017	34,240,815
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests	$ 23	20,852	127		21,002	(21,002)
Redemption of noncontrolling interests (in shares)	2,261,068
Issuance of stock awards, net of forfeitures (in shares)	188,152
Exercise of stock options		310			310		310
Exercise of stock options (in shares)	18,656
Share-based compensation	$ 2	12,657			12,659		12,659
Dividends and distributions				(147,285)	(147,285)	(52,456)	(199,741)
Net income				77,922	77,922	28,841	106,763
Other comprehensive income			(3,073)		(3,073)	(929)	(4,002)
Balance at Dec. 31, 2018	$ 363	491,314	(2,193)	(246,929)	242,555	92,078	334,633
Balance (in shares) at Dec. 31, 2018	36,708,691
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			(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
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests	$ 48	25,817	(3,189)		22,676	(22,676)
Redemption of noncontrolling interests (in shares)	4,854,873
Issuance of stock awards, net of forfeitures (in shares)	186,074
Exercise of stock options		483			483		483
Exercise of stock options (in shares)	26,164
Share-based compensation	$ 1	16,971			16,972		16,972
Dividends and distributions				(202,710)	(202,710)	(35,141)	(237,851)
Net income				79,309	79,309	15,308	94,617
Other comprehensive income			(11,311)		(11,311)	(4,320)	(15,631)
Balance at Dec. 31, 2020	$ 422	$ 555,595	$ (20,526)	$ (471,910)	$ 63,581	$ 8,897	$ 72,478
Balance (in shares) at Dec. 31, 2020	42,768,153

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 94,617	$ 99,037	$ 106,763
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	168,915	152,925	141,633
Amortization of above/below market leases	(139)	(254)	(580)
Amortization of deferred financing costs and hedge amortization	4,128	3,338	2,370
Share-based compensation	15,843	14,384	12,038
Bad debt expense	678	291	(229)
Changes in operating assets and liabilities:
Accounts receivable	(519)	(2,836)	10,923
Deferred rent receivable	(1,780)	4,827	(3,124)
Initial direct costs	(17,729)	(13,422)	(11,672)
Other assets	(3,069)	(8,132)	(4,079)
Accounts payable and accrued expenses	5,365	8,208	3,502
Unearned revenue, prepaid rent and other liabilities	3,750	(9,097)	(1,275)
Operating leases	1,020	2,316	2,491
Net cash provided by operating activities	271,080	251,585	258,761
CASH FLOWS FROM INVESTING ACTIVITIES
Tenant improvements	(6,227)	(4,014)	(5,247)
Real estate improvements	(261,426)	(342,187)	(239,715)
Business combinations and asset acquisitions		(26,060)	(10,681)
Net cash used in investing activities	(267,653)	(372,261)	(255,643)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercise of stock options	483	364	310
Proceeds from revolving credit facility	257,500	397,298	253,036
Payments on revolving credit facility	(171,500)	(546,299)	(211,036)
Proceeds from unsecured debt	150,000	500,000	150,000
Payments of loan fees and costs	(1,111)	(6,337)	(5,038)
Dividends and distributions	(236,304)	(223,901)	(193,038)
Net cash (used in) provided by financing activities	(932)	121,125	(5,766)
Net change in cash and cash equivalents	2,495	449	(2,648)
Cash and cash equivalents, beginning of period	3,048	2,599	5,247
Cash and cash equivalents, end of period	5,543	3,048	2,599
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest, net of capitalized amounts	45,460	31,123	32,579
Cash paid for operating lease liabilities	26,936	25,206	21,994
NON-CASH INVESTING AND FINANCING ACTIVITY
Construction costs payable capitalized to real estate	15,324	64,776	41,328
Accrual of dividends and distributions	63,878	62,332	55,679
NON-CASH OPERATING ACTIVITY
Lease liabilities arising from obtaining right-of-use assets	$ 19,744	$ 1,040	$ 114,989

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2020
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, 2020, the Company owned an 87.7% common interest in our Operating Partnership, and affiliates of The Carlyle Group and others owned a 12.3% 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, 2020
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation and Basis of Presentation The accompanying consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation. Recently Adopted Accounting Pronouncements Fair Value Measurement ​ In August 2018, the Financial Accounting Standards Board (“FASB”) issued guidance codified in Accounting Standards Update (“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 Financial Instruments – Credit Losses In June 2016, the FASB issued guidance codified in ASU 2016-13, Financial Instruments – Credit Losses credit losses” model, which requires companies to estimate credit losses immediately upon exposure. The guidance applies to financial assets measured at amortized cost, including financing receivables and trade receivables. In November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments – Credit Losses Recent Accounting Pronouncements Not Yet Adopted ​ Reference Rate Reform ​ In March 2020, the FASB issued guidance codified in ASU 2020-04, Reference Rate Reform 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 $12.8 million, $13.6 million and $5.6 million for the years ended December 31, 2020, 2019, and 2018, 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 are 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 impairment loss related to these intangible assets was recognized for the years ended December 31, 2020, 2019, or 2018. 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, 2020, and 2019, 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, 2020, 2019, or 2018. 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 $13.2 million, $14.2 million, and $15.6 million for the years ended December 31, 2020, 2019, and 2018, 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, 2020, and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2020 2019 Internal sales commissions ​ $ 16,453 ​ $ 15,064 ​ Third party commissions ​ ​ 13,316 ​ ​ 10,845 ​ Other ​ ​ 357 ​ ​ 462 ​ Total ​ $ 30,126 ​ $ 26,371 ​ ​ 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, 2020, 2019, and 2018, no impairment of long-lived assets was recognized in the consolidated financial statements. Derivative Instruments and Hedging Activities We reflect all derivative instruments at fair value as either assets or liabilities on the consolidated balance sheets. For those derivative instruments that are designated and qualify as hedging instruments, we record the gain or loss on the hedging instruments as a component of accumulated other comprehensive income or loss. For derivatives that do not meet the criteria for hedge accounting, changes in fair value are immediately recognized within net income. See additional discussion in Note 9 Derivatives and Hedging Activities. Internal-Use Software We recognize internal-use software development costs based on the development stage of the project and nature of the cost. Internal and external costs incurred during the preliminary project stage are expensed as they are incurred. Internal and external costs incurred to develop internal-use software during the application development stage are capitalized. Internal and external training costs and maintenance costs during the post-implementation-operation stage are expensed as incurred. Completed projects are placed into service and amortized over the estimated useful life of the software. For the years ended December 31, 2020, 2019, and 2018, no impairment of internal-use software was recognized in the consolidated financial statements. Revenue Recognition Rental, Power, and Related Revenue We derive our revenues from leases with customers for data center and office and light-industrial space. Our leases include rental revenue lease components and nonlease revenue components, such as power and tenant reimbursements. We have elected to combine all of our nonlease revenue components that have the same pattern of transfer as the related operating lease component into a single combined lease component. ​ Our leases with customers are classified as operating leases and rental revenue is recognized on a straight-line basis over the customer lease term. Occasionally, our customer leases include options to extend or terminate the lease agreements. We do not include any of these extension or termination options in a customer’s lease term for lease classification purposes or for recognizing rental revenue unless we are reasonably certain the customer will exercise these extension or termination options. The excess of rents recognized over amounts contractually due pursuant to the underlying leases is recorded as deferred rent receivable within other assets on our consolidated balance sheets. ​ In general, we provide two power products for our data center leased space, consisting of a fixed (breakered-amperage) and a variable (sub-metered) model. Customer power arrangements are coterminous with the customer’s underlying lease and have the same pattern of transfer over the lease term and are therefore combined with lease revenue within our consolidated statements of operations. For 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, power is provided to our customers, and 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 the collectability of 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 not be probable. At December 31, 2020, and 2019, the allowance for doubtful accounts totaled $0.5 million and $0.4 million, respectively, on the consolidated balance sheets. ​ Lessee Accounting We determine if an arrangement is a lease at inception. Our operating lease agreements are primarily for real estate space and are included within operating lease 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, 2020, and 2019, the amount accrued for asset retirement and environmental remediation obligations included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.8 million and $1.7 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, 2020, and 2019, 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, 2020
Acquisitions
Acquisitions	3. Acquisitions 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. During the year ended December 31, 2020, we placed 54,798 square feet into service as part of the completion of phase 1. 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. In the fourth quarter of 2020, CoreSite made the decision to exit and vacate the leased space assumed through the acquisition of U.S. Colo by the end of 2021. 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 plan to develop an approximately 200,000 NRSF turn-key data center building on the acquired land parcel, which we refer to as SV9.

Investment in Real Estate

Investment in Real Estate	12 Months Ended
	Dec. 31, 2020
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, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Buildings and ​ Construction in ​ ​ ​ ​ Market Land Improvements Progress Total Cost Boston ​ $ 5,154 ​ $ 131,037 ​ $ 4,221 ​ $ 140,412 ​ Chicago (1) ​ ​ 7,059 ​ ​ 183,282 ​ ​ 58,883 ​ ​ 249,224 ​ Denver ​ ​ — ​ ​ 35,527 ​ ​ 955 ​ ​ 36,482 ​ Los Angeles (1) ​ ​ 22,127 ​ ​ 451,999 ​ ​ 68,052 ​ ​ 542,178 ​ Miami ​ ​ 728 ​ ​ 14,679 ​ ​ 10 ​ ​ 15,417 ​ New York ​ ​ 3,574 ​ ​ 213,051 ​ ​ 39,228 ​ ​ 255,853 ​ Northern Virginia ​ ​ 21,856 ​ ​ 408,534 ​ ​ 106,318 ​ ​ 536,708 ​ San Francisco Bay (1) ​ ​ 44,236 ​ ​ 835,427 ​ ​ 41,744 ​ ​ 921,407 ​ Total ​ $ 104,734 ​ $ 2,273,536 ​ $ 319,411 ​ $ 2,697,681 ​ ​ (1) Refer to Note 3 – Acquisitions for further detail on CH2, U.S. Colo, and SV9 acquisitions. ​ The following is a summary of the properties owned or leased by market at December 31, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Market Land Buildings and Construction in Total Cost ​ Boston ​ $ 5,154 ​ $ 119,227 ​ $ 931 ​ $ 125,312 ​ Chicago ​ ​ 5,493 ​ ​ 115,699 ​ ​ 100,118 ​ ​ 221,310 ​ Denver ​ ​ — ​ ​ 32,659 ​ ​ 2,461 ​ ​ 35,120 ​ Los Angeles ​ ​ 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 ​ ​ 39,961 ​ ​ 776,642 ​ ​ 72,763 ​ ​ 889,366 ​ Total ​ $ 94,593 ​ $ 1,989,731 ​ $ 394,474 ​ $ 2,478,798 ​ ​

Other Assets

Other Assets	12 Months Ended
	Dec. 31, 2020
Other Assets.
Other Assets	​ 5. Other Assets Other assets consisted of the following, net of amortization and depreciation, if applicable for each line item, as of December 31, 2020, and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2020 2019 Deferred rent receivable ​ $ 40,115 ​ $ 38,335 ​ Initial direct costs ​ ​ 30,126 ​ ​ 26,371 ​ Internal-use software ​ ​ 13,403 ​ ​ 16,747 ​ Prepaid expenses ​ ​ 11,572 ​ ​ 7,675 ​ Corporate furniture, fixtures and equipment ​ ​ 4,813 ​ ​ 4,848 ​ Deferred financing costs - revolving credit facility ​ ​ 2,382 ​ ​ 3,148 ​ Other ​ ​ 683 ​ ​ 3,958 ​ Total ​ $ 103,094 ​ $ 101,082 ​ ​ ​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2021 ​ $ 11,819 ​ 2022 ​ ​ 7,835 ​ 2023 ​ ​ 4,084 ​ 2024 ​ ​ 1,890 ​ 2025 ​ ​ 1,438 ​ Thereafter ​ ​ 3,060 ​ Total ​ $ 30,126 ​ ​

Leases

Leases	12 Months Ended
	Dec. 31, 2020
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 15 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 at each December 31, 2020, and December 31, 2019. The weighted-average discount rate was 4.9% at each date. ​ During the year ended December 31, 2020, we extended the term of approximately 25,000 NRSF at our existing DC2 data center from July 2028 to July 2035 . As a result of this extension, we remeasured the lease liability and adjusted the ROU asset by approximately $7.0 million. In addition, we extended the term of approximately 25,000 NRSF at our existing DC1 data center from May 2021 to March 2032 . As a result of this extension, we remeasured the lease liability and the ROU asset by approximately $11.8 million. ​ 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, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Lease expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating lease expense ​ $ 27,982 ​ $ 26,566 ​ $ 22,927 ​ Variable lease expense ​ ​ 8,565 ​ ​ 6,058 ​ ​ 4,769 ​ Rent expense ​ $ 36,547 ​ $ 32,624 ​ $ 27,696 ​ ​ The future maturity of the operating lease liabilities in effect at December 31, 2020 is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ Operating ​ Year Ending December 31, Leases ​ 2021 ​ $ 28,203 ​ 2022 ​ ​ 26,454 ​ 2023 ​ ​ 25,335 ​ 2024 ​ ​ 24,938 ​ 2025 ​ ​ 24,163 ​ Thereafter ​ ​ 104,082 ​ Total lease payments ​ $ 233,175 ​ Less imputed interest ​ ​ (43,771) ​ Total ​ $ 189,404 ​ ​

Lease Revenue

Lease Revenue	12 Months Ended
	Dec. 31, 2020
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, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Lease revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum lease revenue ​ $ 432,781 ​ $ 409,622 ​ $ 390,691 ​ Variable lease revenue ​ ​ 89,970 ​ ​ 87,354 ​ ​ 83,992 ​ Total lease revenue ​ $ 522,751 ​ $ 496,976 ​ $ 474,683 ​ ​ The future minimum lease payments to be received under noncancelable data center, office, light-industrial, and other operating leases in effect at December 31, 2020, are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ Minimum ​ Year Ending December 31, Lease Payments ​ 2021 ​ $ 381,095 ​ 2022 ​ ​ 267,866 ​ 2023 ​ ​ 177,704 ​ 2024 ​ ​ 127,030 ​ 2025 ​ ​ 102,744 ​ Thereafter ​ ​ 199,977 ​ Total ​ $ 1,256,416 ​ ​

Debt

Debt	12 Months Ended
	Dec. 31, 2020
Debt
Debt	8. Debt A summary of outstanding indebtedness as of December 31, 2020, and 2019, is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maturity ​ December 31, ​ December 31, ​ ​ Interest Rate Date 2020 2019 Revolving credit facility (1) ​ 1.39% and 3.01% at December 31, 2020, and December 31, 2019, respectively ​ November 8, 2023 ​ $ 148,500 ​ $ 62,500 ​ 2022 Senior unsecured term loan (2)(3) ​ 1.76% and 2.96% at December 31, 2020, and December 31, 2019, respectively ​ April 19, 2022 ​ ​ 200,000 ​ ​ 200,000 ​ 2023 Senior unsecured notes ​ 4.19% at December 31, 2020, and December 31, 2019, respectively ​ June 15, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured term loan (2)(3) ​ 2.86% and 3.44% at December 31, 2020, and December 31, 2019, respectively ​ April 19, 2024 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured notes ​ 3.91% at December 31, 2020, and December 31, 2019, respectively ​ April 20, 2024 ​ ​ 175,000 ​ ​ 175,000 ​ 2025 Senior unsecured term loan (2)(3) ​ 2.32% and 2.81% at December 31, 2020, and December 31, 2019, respectively ​ April 1, 2025 ​ ​ 350,000 ​ ​ 350,000 ​ 2026 Senior unsecured notes (2) ​ 4.52% at December 31, 2020, and December 31, 2019, respectively ​ April 17, 2026 ​ ​ 200,000 ​ ​ 200,000 ​ 2027 Senior unsecured notes ​ 3.75% at December 31, 2020 ​ May 6, 2027 ​ ​ 150,000 ​ ​ — ​ 2029 Senior unsecured notes ​ 4.31% at December 31, 2020, and December 31, 2019, respectively ​ April 17, 2029 ​ ​ 200,000 ​ ​ 200,000 ​ Total principal outstanding ​ ​ ​ ` ​ ​ 1,723,500 ​ ​ 1,487,500 ​ Unamortized deferred financing costs ​ ​ ​ ​ ​ ​ (7,589) ​ ​ (9,098) ​ Total debt ​ ​ ​ ​ ​ $ 1,715,911 ​ $ 1,478,402 ​ (1) Borrowings under the revolving credit facility bear interest at a variable rate per annum equal to either (i) LIBOR plus 125 basis points to 185 basis points, or (ii) a base rate plus 25 basis points to 85 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2020, our Operating Partnership’s leverage ratio was 33.6% and the interest rate was LIBOR plus 125 basis points. ​ (2) 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, 2020, and 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. ​ (3) Borrowings under the senior unsecured term loans 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, 2020, our Operating Partnership’s leverage ratio was 33.6% and the interest rate was LIBOR plus 120 basis points. ​ Debt Instruments 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”), 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. The revolving credit facility includes 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. Borrowings under the revolving credit facility have been amended to bear interest at a variable rate per annum equal to either (i) LIBOR plus 125 basis points to 185 basis points, or (ii) a base rate plus 25 basis points to 85 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2020, our Operating Partnership’s interest rate was LIBOR plus 125 basis points. The total amount available for borrowing under the revolving credit facility, is equal to the lesser of $450.0 million or the availability calculated based on our unencumbered asset pool. As of December 31, 2020, the borrowing capacity was $450.0 million. As of December 31, 2020, $148.5 million was borrowed and outstanding, $6.1 million was outstanding under letters of credit, and therefore $295.4 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, 2020, was 33.6% ; ● a maximum secured debt ratio (defined as total consolidated secured debt to total gross asset value) of 40%, which, as of December 31, 2020, 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, 2020 was 6.0 to 1.0. 2022 Senior Unsecured Term Loan ​ On April 19, 2017, our Operating Partnership and certain subsidiaries entered into an Amended and Restated Term Loan Agreement (the “2022 Term Loan”). As amended and restated, the 2022 Term Loan (i) increased the total commitments to $200 million, (ii) extended the maturity of the term loan to April 19, 2022, (iii) amended 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 permitted the issuance of the 2024 Notes (the “2024 Notes”). ​ The 2022 Term Loan ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ Borrowings under the 2022 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, 2020, our Operating Partnership’s leverage ratio was 33.6% and the interest rate was LIBOR plus 120 basis points. ​ 2024 Senior Unsecured Term Loan ​ ​ The 2024 Term Loan ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ 2025 Senior Unsecured Term Loan ​ ​ The 2025 Term Loan ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ 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, 2020, our Operating Partnership’s leverage ratio was 33.6% 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. ​ On June 12, 2018, the 2023 Note Purchase Agreement was amended to, among other things, conform to the same financial covenants as described below. In addition, certain additional financial covenants in the Amended and Restated Credit Agreement were automatically incorporated into the 2023 Note Purchase Agreement. The 2023 Notes ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. 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. ​ On June 12, 2018, the 2023 Note Purchase Agreement was amended to, among other things, conform to the same financial covenants as described below. In addition, certain additional financial covenants in the Amended and Restated Credit Agreement were automatically incorporated into the 2024 Note Purchase Agreement. ​ The 2024 Notes ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ 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 ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ 2027 Senior Unsecured Notes ​ On May 6, 2020, our Operating Partnership agreed to issue an aggregate principal amount of $150 million, 3.75% Series C senior unsecured notes due May 6, 2027 (the “2027 Notes”), in a private placement to certain accredited investors. An aggregate principal amount of $100 million of the 2027 Notes was issued on May 6, 2020. The remaining $50 million was issued on July 14, 2020. The terms of the 2027 Notes are governed by a note purchase agreement, dated May 6, 2020 (the “2027 Note Purchase Agreement”), by and among us, our Operating Partnership, and the purchasers of the 2027 Notes. ​ Interest is payable semiannually based on a fixed rate, on the 15 th ​ Our Operating Partnership may prepay all or a portion of the 2027 Notes upon notice to the holders for 100% of the principal amount so prepaid plus a make-whole premium as set forth in the 2027 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2027 Notes would have the right to require that the Operating Partnership purchase 100% of such holders’ 2027 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 2027 Notes ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ 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. An aggregate principal amount of $125 million of the 2029 Notes was issued on April 17, 2019. The remaining $75 million of the 2029 Notes was issued on July 17, 2019. The terms of the 2029 Notes are governed by the 2026 Note Purchase Agreement, by and among our Operating Partnership, the Company and the purchasers of the 2029 Notes. ​ 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 ranks pari passu with the each of the senior term loans, each of the senior notes, and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. ​ Debt Covenants ​ All of our debt instruments contain certain financial covenants and other customary restrictive covenants, including limitations on transactions with affiliates, merger, consolidation, and sales of assets, liens and subsidiary indebtedness. Our financial covenants include maximum consolidated total unsecured indebtedness to unencumbered asset pool availability, minimum consolidated tangible net worth, a maximum ratio of consolidated total indebtedness to gross asset value, a minimum ratio of adjusted consolidated EBITDA to consolidated fixed charges, and a maximum ratio of secured indebtedness to gross asset value. As of December 31, 2020, we were in compliance with all of the financial covenants. ​ ​ ​ ​ Debt Maturities The following table summarizes when our debt currently becomes due (in thousands): ​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2021 ​ $ — ​ 2022 ​ ​ 200,000 ​ 2023 ​ ​ 298,500 ​ 2024 ​ ​ 325,000 ​ 2025 ​ ​ 350,000 ​ Thereafter ​ ​ 550,000 ​ Total principal outstanding ​ ​ 1,723,500 ​ Unamortized deferred financing costs ​ ​ (7,589) ​ Total debt, net ​ $ 1,715,911 ​ ​

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2020
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, 2020, and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Notional Amount ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value (Level 2) (1) ​ December 31, ​ December 31, ​ Type of ​ ​ ​ ​ ​ ​ Effective ​ Expiration ​ December 31, ​ December 31, ​ 2020 ​ 2019 ​ Derivative ​ Index ​ Strike Rate ​ ​ Date ​ Date ​ 2020 ​ 2019 ​ $ — ​ $ 75,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 1.43 % ​ 5/5/2015 ​ 5/5/2020 (2) $ — ​ $ 67 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 2.72 ​ ​ 5/5/2018 ​ 4/5/2023 ​ ​ (4,388) ​ ​ (2,819) ​ ​ 100,000 ​ ​ 100,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 1.59 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ (5,657) ​ ​ 55 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 1.59 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ (4,243) ​ ​ 41 ​ ​ 200,000 ​ ​ — ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 0.56 ​ ​ 3/5/2020 ​ 4/19/2022 ​ ​ (1,109) ​ ​ — ​ ​ 75,000 ​ ​ — ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 0.61 ​ ​ 3/5/2020 ​ 10/5/2023 ​ ​ (930) ​ ​ — ​ ​ 175,000 ​ ​ — ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 0.64 ​ ​ 3/5/2020 ​ 10/1/2024 ​ ​ (2,773) ​ ​ — ​ $ 700,000 ​ $ 325,000 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ (19,100) ​ $ (2,656) ​ (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. ​ (2) On May 5, 2020, the $75 million five-year interest rate swap agreement, which reduced our variability in cash flows relating to interest payments based on one-month LIBOR variable debt, expired. ​ 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, ​ ​ 2020 ​ 2019 ​ 2018 ​ Cash Flow Hedges ​ ​ ​ ​ ​ ​ ​ ​ ​ Unrealized loss recognized in other comprehensive income $ (20,647) ​ $ (5,038) ​ $ (3,676) ​ (Gain) loss reclassified from other comprehensive income to interest expense ​ 5,016 ​ ​ 212 ​ ​ (326) ​ Interest expense ​ 45,086 ​ ​ 41,712 ​ ​ 35,526 ​ ​ 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, 2021, we estimate that $7.5 million will be reclassified as an increase to interest expense.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2020
Stockholders' Equity
Stockholders' Equity	10. Stockholders’ Equity At December 31, 2020, 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, 2020, we had 42.8 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 $0.5 million, $0.4 million, and $0.3 million for the years ended December 31, 2020, 2019, and 2018, 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 212,238, 188,741, and 206,808 during the years ended December 31, 2020, 2019, and 2018, respectively. See Note 12 – Equity Incentive Award Plan, for additional information on our 2010 Plan. During the years ended December 31, 2020, 2019, and 2018, 4,850,172, 800,000 and 2,250,000 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, 2020, 2019, and 2018, 4,701, 3,610, and 11,068 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. Dividends In 2020, 2019, and 2018, we paid all dividends in cash. The following summarizes the taxability of our common stock dividends per share for the years then ended: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ Record Date ​ 2020 ​ 2019 ​ 2018 ​ Common Stock: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Ordinary income (1) ​ $ 3.14 ​ $ 3.07 ​ $ 3.09 ​ Qualified dividend ​ ​ — ​ ​ — ​ ​ — ​ Capital gains ​ ​ — ​ ​ — ​ ​ — ​ Return of capital ​ ​ 1.74 ​ ​ 1.57 ​ ​ 0.93 ​ Total dividend (2) ​ $ 4.88 ​ $ 4.64 ​ $ 4.02 ​ (1) All taxable 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) Distributions made during calendar years ended December 31, 2020, 2019, and 2018 were subject to the tax treatment applicable to the year in which they are distributed. ​ 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. Our federal tax return for the year ended December 31, 2020 has not been filed. The taxability information presented for our common stock dividends paid in 2020 is based upon management’s estimates.

Noncontrolling Interests - Oper

Noncontrolling Interests - Operating Partnership	12 Months Ended
	Dec. 31, 2020
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, 2020 ​ December 31, 2019 ​ ​ Number of Units Percentage of Total ​ Number of Units Percentage of Total ​ CoreSite Realty Corporation ​ 42,276,221 ​ 87.7 % 37,244,987 ​ 77.6 % Noncontrolling interests ​ 5,916,785 ​ 12.3 ​ 10,771,658 ​ 22.4 ​ Total ​ 48,193,006 ​ 100.0 % 48,016,645 ​ 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, 2020, we issued 176,361 shares of common stock related to employee compensation arrangements and therefore an equivalent number of common Operating Partnership units were issued to us by our Operating Partnership. Holders of common Operating Partnership units of record as of March 31, 2020, June 30, 2020, September 30, 2020, and December 31, 2020, received quarterly distributions of $1.22 per unit, $1.22 per unit, $1.22 per unit, and $1.23 per unit, respectively, payable in correlation with declared dividends on shares of our common stock. During the years ended December 31, 2020, 2019, and 2018, 4,854,873, 803,610, and 2,261,068, 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 $22.7 million, $5.2 million, and $21.0 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, 2020, 2019, and 2018, respectively. The redemption value of the noncontrolling interests at December 31, 2020, was $741.3 million based on the closing price of the Company’s common stock of $125.28 on the last trading day prior to that date.

Equity Incentive Plan

Equity Incentive Plan	12 Months Ended
	Dec. 31, 2020
Equity Incentive 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 awards, restricted stock units, 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, 2020, 2,480,212 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, 2020, 2019, and 2018 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted ​ Weighted ​ Aggregate 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 ​ Granted ​ — ​ ​ — ​ ​ ​ ​ ​ Exercised ​ (26,164) ​ ​ 19.53 ​ ​ ​ $ 2.7 million ​ Forfeited ​ — ​ ​ — ​ ​ ​ ​ ​ Expired ​ — ​ ​ — ​ ​ ​ ​ ​ Options outstanding, December 31, 2020 ​ 5,582 ​ $ 19.76 ​ 0.7 years ​ $ 0.6 million ​ ​ As of December 31, 2017, all stock option awards were fully vested and exercisable. ​ Restricted Stock Awards and Units Restricted stock awards (“RSAs”) and restricted stock units (“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 RSAs 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 RSAs and RSUs are amortized on a straight-line basis to expense over the vesting period. The following table sets forth the number of unvested RSAs and RSUs and the weighted-average fair value of these awards at the date of grant: ​ ​ ​ ​ ​ ​ ​ ​ ​ Restricted Stock Weighted 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 ​ Granted ​ 159,807 ​ ​ 109.81 ​ Forfeited ​ (23,980) ​ ​ 102.81 ​ Vested ​ (125,405) ​ ​ 90.77 ​ Unvested balance, December 31, 2020 ​ 302,795 ​ $ 105.35 ​ ​ As of December 31, 2020, total unearned compensation on RSAs and RSUs was approximately $21.5 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 2018, 2019, and 2020. 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, 2019 ​ 34,624 ​ 171,351 ​ 102,989 ​ $ 107.84 ​ Granted ​ 10,210 ​ 71,488 ​ 40,852 ​ ​ 125.02 ​ Performance adjustment (1) ​ 38,047 ​ (7,904) ​ 15,073 ​ ​ — ​ Forfeited ​ (1,049) ​ (4,878) ​ (2,964) ​ ​ 107.63 ​ Vested ​ (32,524) ​ (32,524) ​ (32,524) ​ ​ 105.55 ​ Unvested balance, December 31, 2020 ​ 49,308 ​ 197,533 ​ 123,426 ​ $ 113.96 ​ (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, 2020, total unearned compensation on PSAs was approximately $6.0 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, 2020, 2019, and 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2020 2019 2018 Expected term (in years) ​ 2.82 ​ ​ 2.82 ​ 2.82 ​ Expected volatility ​ 24.00 % ​ 24.09 % 24.15 % Expected annual dividend (1) ​ — ​ ​ — ​ — ​ Risk-free rate ​ 0.56 % ​ 2.48 % 2.24 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2020
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, ​ ​ 2020 2019 2018 Net income attributable to common shares ​ $ 79,309 ​ $ 75,840 ​ $ 77,922 ​ Weighted-average common shares outstanding - basic ​ ​ 40,453,438 ​ ​ 36,766,310 ​ ​ 34,957,244 ​ Effect of potentially dilutive common shares: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options ​ ​ 16,568 ​ ​ 36,685 ​ ​ 47,338 ​ Unvested awards ​ ​ 164,318 ​ ​ 140,878 ​ ​ 132,913 ​ Weighted-average common shares outstanding - diluted ​ ​ 40,634,324 ​ ​ 36,943,873 ​ ​ 35,137,495 ​ Net income per share attributable to common shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ $ 1.96 ​ $ 2.06 ​ $ 2.23 ​ Diluted ​ $ 1.95 ​ $ 2.05 ​ $ 2.22 ​ ​ In the calculations above, we have excluded weighted-average potentially dilutive securities of 3,933, 2,546, and 132,749 for the years ended December 31, 2020, 2019, and 2018, respectively, as their effect would have been antidilutive.

Employee Benefit Plan

Employee Benefit Plan	12 Months Ended
	Dec. 31, 2020
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, 2020, 2019, and 2018, our contributions were $1.9 million, $1.7 million, and $1.2 million, respectively.

Estimated Fair Value of Financi

Estimated Fair Value of Financial Instruments	12 Months Ended
	Dec. 31, 2020
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, 2020, 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 $1.7 billion and $1.5 billion as of December 31, 2020 and 2019, 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 lease 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, 2020
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, 2020, we had open commitments related to construction contracts of approximately $39.9 million. Additionally, we have commitments related to telecommunications capacity used to connect data centers located within the same market or geographical area, power usage, including fuel cell commitments, and company-wide improvements that are ancillary to revenue generation. At December 31, 2020, we had open commitments related to these contracts of approximately $71.0 million of which $7.8 million is scheduled to be met during the year ended December 31, 2021. In the ordinary course of business, we are subject to claims and administrative proceedings. We are not presently party to any proceeding, which we believe to be material or which we would expect to have, individually or in the aggregate, a material adverse effect on our business, financial condition, cash flows or results of operations. The outcome of litigation and administrative proceedings is inherently uncertain. Therefore, if one or more legal or administrative matters are resolved against us in a reporting period for amounts in excess of management’s expectations, our financial condition, cash flows or results of operations for that reporting period could be materially adversely affected.

Quarterly Financial Information

Quarterly Financial Information (unaudited)	12 Months Ended
	Dec. 31, 2020
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, 2020, and 2019 (in thousands except share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ ​ December 31, ​ September 30, ​ June 30, ​ March 31, ​ ​ 2020 2020 2020 2020 Revenue ​ $ 154,938 ​ $ 153,981 ​ $ 150,543 ​ $ 147,362 ​ Operating income ​ ​ 34,352 ​ ​ 35,526 ​ ​ 35,693 ​ ​ 34,188 ​ Net income ​ ​ 22,409 ​ ​ 24,132 ​ ​ 25,088 ​ ​ 22,988 ​ Net income attributable to common shares ​ ​ 19,658 ​ ​ 21,132 ​ ​ 20,671 ​ ​ 17,848 ​ Net income per share attributable to common shares - basic ​ $ 0.46 ​ $ 0.50 ​ $ 0.52 ​ $ 0.48 ​ Net income per share attributable to common shares - diluted ​ $ 0.46 ​ $ 0.50 ​ $ 0.52 ​ $ 0.48 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ ​

Schedule III Real Estate and Ac

Schedule III Real Estate and Accumulated Depreciation	12 Months Ended
	Dec. 31, 2020
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, 2020 (in thousands) ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Gross Amount Carried at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Initial Cost ​ Costs Capitalized ​ December 31, 2020 ​ Accumulated ​ Year ​ ​ ​ ​ ​ ​ ​ ​ Building and ​ Subsequent to ​ ​ ​ Building and ​ ​ ​ ​ Depreciation at ​ Acquired ​ Property Name Location Land Improvements Acquisition Land Improvements Total December 31, 2020 or Leased BO1 ​ Somerville, MA ​ $ 6,100 ​ $ 26,748 ​ $ 107,564 ​ $ 5,154 ​ $ 135,258 ​ $ 140,412 ​ $ 73,967 ​ 2007 ​ CH1 ​ Chicago, IL ​ ​ 5,493 ​ ​ 49,522 ​ ​ 67,728 ​ ​ 5,493 ​ ​ 117,250 ​ ​ 122,743 ​ ​ 61,401 ​ 2010 ​ CH2 ​ Chicago, IL ​ ​ 4,383 ​ ​ — ​ ​ 122,098 ​ ​ 1,566 ​ ​ 124,915 ​ ​ 126,481 ​ ​ 2,432 ​ 2018 ​ DC1 ​ Washington, DC ​ ​ — ​ ​ 4,797 ​ ​ 5,398 ​ ​ — ​ ​ 10,195 ​ ​ 10,195 ​ ​ 8,412 ​ 2010 ​ DC2 ​ Washington, DC ​ ​ — ​ ​ — ​ ​ 23,920 ​ ​ — ​ ​ 23,920 ​ ​ 23,920 ​ ​ 4,648 ​ 2017 ​ DE1 ​ Denver, CO ​ ​ — ​ ​ 37 ​ ​ 33,377 ​ ​ — ​ ​ 33,414 ​ ​ 33,414 ​ ​ 13,065 ​ 2012 ​ DE2 ​ Denver, CO ​ ​ — ​ ​ 4 ​ ​ 3,064 ​ ​ — ​ ​ 3,068 ​ ​ 3,068 ​ ​ 1,314 ​ 2012 ​ LA1 ​ Los Angeles, CA ​ ​ — ​ ​ 41,657 ​ ​ 64,257 ​ ​ — ​ ​ 105,914 ​ ​ 105,914 ​ ​ 67,543 ​ 2010 ​ LA2 ​ Los Angeles, CA ​ ​ 18,672 ​ ​ 94,114 ​ ​ 181,631 ​ ​ 18,672 ​ ​ 275,745 ​ ​ 294,417 ​ ​ 115,786 ​ 2010 ​ LA3 ​ Los Angeles, CA ​ ​ 9,795 ​ ​ — ​ ​ 129,116 ​ ​ 3,455 ​ ​ 135,456 ​ ​ 138,911 ​ ​ 888 ​ 2010 ​ LA4 ​ Los Angeles, CA ​ ​ — ​ ​ 2,533 ​ ​ 403 ​ ​ — ​ ​ 2,936 ​ ​ 2,936 ​ ​ 1,630 ​ 2018 ​ MI1 ​ Miami, FL ​ ​ 728 ​ ​ 9,325 ​ ​ 5,364 ​ ​ 728 ​ ​ 14,689 ​ ​ 15,417 ​ ​ 5,841 ​ 2010 ​ NY1 ​ New York, NY ​ ​ — ​ ​ — ​ ​ 46,274 ​ ​ — ​ ​ 46,274 ​ ​ 46,274 ​ ​ 34,027 ​ 2007 ​ NY2 ​ Secaucus, NJ ​ ​ 4,952 ​ ​ 18,408 ​ ​ 186,219 ​ ​ 3,574 ​ ​ 206,005 ​ ​ 209,579 ​ ​ 47,551 ​ 2013 ​ SV1 ​ San Jose, CA ​ ​ 6,863 ​ ​ 91,719 ​ ​ 44,561 ​ ​ 6,863 ​ ​ 136,280 ​ ​ 143,143 ​ ​ 56,546 ​ 2010 ​ SV2 ​ Milpitas, CA ​ ​ 5,086 ​ ​ 5,046 ​ ​ 29,152 ​ ​ 5,086 ​ ​ 34,198 ​ ​ 39,284 ​ ​ 24,996 ​ 2006 ​ SV3 ​ Santa Clara, CA ​ ​ 4,162 ​ ​ 3,482 ​ ​ 51,217 ​ ​ 3,972 ​ ​ 54,889 ​ ​ 58,861 ​ ​ 47,541 ​ 2007 ​ SV4 ​ Santa Clara, CA ​ ​ 4,632 ​ ​ 3,716 ​ ​ 101,126 ​ ​ 4,501 ​ ​ 104,973 ​ ​ 109,474 ​ ​ 61,835 ​ 2007 ​ SV5 ​ Santa Clara, CA ​ ​ 2,572 ​ ​ — ​ ​ 23,653 ​ ​ 2,544 ​ ​ 23,681 ​ ​ 26,225 ​ ​ 4,502 ​ 2007 ​ SV6 ​ Santa Clara, CA ​ ​ 4,741 ​ ​ — ​ ​ 36,868 ​ ​ 4,825 ​ ​ 36,784 ​ ​ 41,609 ​ ​ 4,312 ​ 2007 ​ SV7 ​ Santa Clara, CA ​ ​ 3,793 ​ ​ — ​ ​ 248,523 ​ ​ 3,595 ​ ​ 248,721 ​ ​ 252,316 ​ ​ 59,213 ​ 2007 ​ SV8 ​ Santa Clara, CA ​ ​ 12,158 ​ ​ — ​ ​ 204,949 ​ ​ 12,850 ​ ​ 204,257 ​ ​ 217,107 ​ ​ 14,211 ​ 2017 ​ SV9 ​ Santa Clara, CA ​ ​ 25,767 ​ ​ 407 ​ ​ 7,214 ​ ​ — ​ ​ 33,388 ​ ​ 33,388 ​ ​ 407 ​ 2019 ​ VA1 ​ Reston, VA ​ ​ 6,903 ​ ​ 32,939 ​ ​ 98,080 ​ ​ 6,779 ​ ​ 131,143 ​ ​ 137,922 ​ ​ 90,229 ​ 2007 ​ VA2 ​ Reston, VA ​ ​ 5,197 ​ ​ — ​ ​ 135,875 ​ ​ 5,539 ​ ​ 135,533 ​ ​ 141,072 ​ ​ 52,050 ​ 2007 ​ VA3 ​ Reston, VA ​ ​ 8,529 ​ ​ 26,411 ​ ​ 165,446 ​ ​ 3,866 ​ ​ 196,520 ​ ​ 200,386 ​ ​ 11,676 ​ 2016 ​ Reston Campus Expansion ​ Reston, VA ​ ​ 5,671 ​ ​ 17,538 ​ ​ 4 ​ ​ 5,672 ​ ​ 17,541 ​ ​ 23,213 ​ ​ 1,951 ​ 2016 ​ ​ ​ Total ​ $ 146,197 ​ $ 428,403 ​ $ 2,123,081 ​ $ 104,734 ​ $ 2,592,947 ​ $ 2,697,681 ​ $ 867,975 ​ ​ ​ ​ None of our properties are encumbered. The aggregate cost of the total properties for federal income tax purposes was approximately $2,432.2 million (unaudited) at December 31, 2020. See accompanying report of independent registered public accounting firm. ​ CoreSite Realty Corporation Schedule III Real Estate and Accumulated Depreciation (Continued) December 31, 2020 (in thousands) ​ The following table reconciles the historical cost and accumulated depreciation of our properties for the years ended December 31, 2020, 2019, and 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 2019 2018 Property ​ ​ ​ ​ ​ ​ ​ ​ Balance, beginning of year ​ $ 2,478,798 ​ $ 2,083,205 $ 1,821,217 ​ Additions - property acquisitions ​ ​ — ​ ​ 26,174 ​ ​ 7,474 Additions - improvements ​ ​ 218,883 ​ ​ 369,419 ​ ​ 254,514 ​ Balance, end of year ​ $ 2,697,681 ​ $ 2,478,798 ​ $ 2,083,205 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Accumulated Depreciation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance, beginning of year ​ $ 720,498 ​ $ 590,784 ​ $ 473,141 ​ Additions, net of disposals ​ ​ 147,477 ​ ​ 129,714 ​ ​ 117,643 ​ Balance, end of year ​ $ 867,975 ​ $ 720,498 ​ $ 590,784 ​ ​ 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, 2020
Summary of Significant Accounting Policies
Principles of Consolidation and Basis of Presentation	Principles of Consolidation and Basis of Presentation The accompanying consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation.
Recent Accounting Pronouncements	Recently Adopted Accounting Pronouncements Fair Value Measurement ​ In August 2018, the Financial Accounting Standards Board (“FASB”) issued guidance codified in Accounting Standards Update (“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 Financial Instruments – Credit Losses In June 2016, the FASB issued guidance codified in ASU 2016-13, Financial Instruments – Credit Losses credit losses” model, which requires companies to estimate credit losses immediately upon exposure. The guidance applies to financial assets measured at amortized cost, including financing receivables and trade receivables. In November 2018, the FASB issued ASU 2018-19, Codification Improvements to Topic 326, Financial Instruments – Credit Losses Recent Accounting Pronouncements Not Yet Adopted ​ Reference Rate Reform ​ In March 2020, the FASB issued guidance codified in ASU 2020-04, Reference Rate Reform 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 $12.8 million, $13.6 million and $5.6 million for the years ended December 31, 2020, 2019, and 2018, 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 are 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 impairment loss related to these intangible assets was recognized for the years ended December 31, 2020, 2019, or 2018. 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, 2020, and 2019, 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, 2020, 2019, or 2018.
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 $13.2 million, $14.2 million, and $15.6 million for the years ended December 31, 2020, 2019, and 2018, 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, 2020, and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2020 2019 Internal sales commissions ​ $ 16,453 ​ $ 15,064 ​ Third party commissions ​ ​ 13,316 ​ ​ 10,845 ​ Other ​ ​ 357 ​ ​ 462 ​ Total ​ $ 30,126 ​ $ 26,371 ​
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, 2020, 2019, and 2018, no impairment of long-lived assets was recognized in the consolidated financial statements.
Derivative Instruments and Hedging Activities	Derivative Instruments and Hedging Activities We reflect all derivative instruments at fair value as either assets or liabilities on the consolidated balance sheets. For those derivative instruments that are designated and qualify as hedging instruments, we record the gain or loss on the hedging instruments as a component of accumulated other comprehensive income or loss. For derivatives that do not meet the criteria for hedge accounting, changes in fair value are immediately recognized within net income. See additional discussion in Note 9 Derivatives and Hedging Activities.
Internal-Use Software	Internal-Use Software We recognize internal-use software development costs based on the development stage of the project and nature of the cost. Internal and external costs incurred during the preliminary project stage are expensed as they are incurred. Internal and external costs incurred to develop internal-use software during the application development stage are capitalized. Internal and external training costs and maintenance costs during the post-implementation-operation stage are expensed as incurred. Completed projects are placed into service and amortized over the estimated useful life of the software. For the years ended December 31, 2020, 2019, and 2018, no impairment of internal-use software was recognized in the consolidated financial statements.
Revenue Recognition	Revenue Recognition Rental, Power, and Related Revenue We derive our revenues from leases with customers for data center and office and light-industrial space. Our leases include rental revenue lease components and nonlease revenue components, such as power and tenant reimbursements. We have elected to combine all of our nonlease revenue components that have the same pattern of transfer as the related operating lease component into a single combined lease component. ​ Our leases with customers are classified as operating leases and rental revenue is recognized on a straight-line basis over the customer lease term. Occasionally, our customer leases include options to extend or terminate the lease agreements. We do not include any of these extension or termination options in a customer’s lease term for lease classification purposes or for recognizing rental revenue unless we are reasonably certain the customer will exercise these extension or termination options. The excess of rents recognized over amounts contractually due pursuant to the underlying leases is recorded as deferred rent receivable within other assets on our consolidated balance sheets. ​ In general, we provide two power products for our data center leased space, consisting of a fixed (breakered-amperage) and a variable (sub-metered) model. Customer power arrangements are coterminous with the customer’s underlying lease and have the same pattern of transfer over the lease term and are therefore combined with lease revenue within our consolidated statements of operations. For 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, power is provided to our customers, and 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 the collectability of 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 not be probable. At December 31, 2020, and 2019, the allowance for doubtful accounts totaled $0.5 million and $0.4 million, respectively, on the consolidated balance sheets.
Lessee Accounting	Lessee Accounting We determine if an arrangement is a lease at inception. Our operating lease agreements are primarily for real estate space and are included within operating lease 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, 2020, and 2019, the amount accrued for asset retirement and environmental remediation obligations included in unearned revenue, prepaid rent and other liabilities on the consolidated balance sheets was approximately $1.8 million and $1.7 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, 2020, and 2019, 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, 2020
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, ​ ​ 2020 2019 Internal sales commissions ​ $ 16,453 ​ $ 15,064 ​ Third party commissions ​ ​ 13,316 ​ ​ 10,845 ​ Other ​ ​ 357 ​ ​ 462 ​ Total ​ $ 30,126 ​ $ 26,371 ​

Investment in Real Estate (Tabl

Investment in Real Estate (Tables)	12 Months Ended
	Dec. 31, 2020
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, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Buildings and ​ Construction in ​ ​ ​ ​ Market Land Improvements Progress Total Cost Boston ​ $ 5,154 ​ $ 131,037 ​ $ 4,221 ​ $ 140,412 ​ Chicago (1) ​ ​ 7,059 ​ ​ 183,282 ​ ​ 58,883 ​ ​ 249,224 ​ Denver ​ ​ — ​ ​ 35,527 ​ ​ 955 ​ ​ 36,482 ​ Los Angeles (1) ​ ​ 22,127 ​ ​ 451,999 ​ ​ 68,052 ​ ​ 542,178 ​ Miami ​ ​ 728 ​ ​ 14,679 ​ ​ 10 ​ ​ 15,417 ​ New York ​ ​ 3,574 ​ ​ 213,051 ​ ​ 39,228 ​ ​ 255,853 ​ Northern Virginia ​ ​ 21,856 ​ ​ 408,534 ​ ​ 106,318 ​ ​ 536,708 ​ San Francisco Bay (1) ​ ​ 44,236 ​ ​ 835,427 ​ ​ 41,744 ​ ​ 921,407 ​ Total ​ $ 104,734 ​ $ 2,273,536 ​ $ 319,411 ​ $ 2,697,681 ​ ​ (1) Refer to Note 3 – Acquisitions for further detail on CH2, U.S. Colo, and SV9 acquisitions. ​ The following is a summary of the properties owned or leased by market at December 31, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Market Land Buildings and Construction in Total Cost ​ Boston ​ $ 5,154 ​ $ 119,227 ​ $ 931 ​ $ 125,312 ​ Chicago ​ ​ 5,493 ​ ​ 115,699 ​ ​ 100,118 ​ ​ 221,310 ​ Denver ​ ​ — ​ ​ 32,659 ​ ​ 2,461 ​ ​ 35,120 ​ Los Angeles ​ ​ 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 ​ ​ 39,961 ​ ​ 776,642 ​ ​ 72,763 ​ ​ 889,366 ​ Total ​ $ 94,593 ​ $ 1,989,731 ​ $ 394,474 ​ $ 2,478,798 ​

Other Assets (Tables)

Other Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Other Assets.
Schedule of other assets	Other assets consisted of the following, net of amortization and depreciation, if applicable for each line item, as of December 31, 2020, and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ ​ 2020 2019 Deferred rent receivable ​ $ 40,115 ​ $ 38,335 ​ Initial direct costs ​ ​ 30,126 ​ ​ 26,371 ​ Internal-use software ​ ​ 13,403 ​ ​ 16,747 ​ Prepaid expenses ​ ​ 11,572 ​ ​ 7,675 ​ Corporate furniture, fixtures and equipment ​ ​ 4,813 ​ ​ 4,848 ​ Deferred financing costs - revolving credit facility ​ ​ 2,382 ​ ​ 3,148 ​ Other ​ ​ 683 ​ ​ 3,958 ​ Total ​ $ 103,094 ​ $ 101,082 ​
Estimated amortization of deferred leasing costs for each of the five succeeding fiscal years	​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2021 ​ $ 11,819 ​ 2022 ​ ​ 7,835 ​ 2023 ​ ​ 4,084 ​ 2024 ​ ​ 1,890 ​ 2025 ​ ​ 1,438 ​ Thereafter ​ ​ 3,060 ​ Total ​ $ 30,126 ​

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases
Schedule of components of lease expense	The components of lease expense were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Lease expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating lease expense ​ $ 27,982 ​ $ 26,566 ​ $ 22,927 ​ Variable lease expense ​ ​ 8,565 ​ ​ 6,058 ​ ​ 4,769 ​ Rent expense ​ $ 36,547 ​ $ 32,624 ​ $ 27,696 ​
Schedule of future maturity of operating lease liability	The future maturity of the operating lease liabilities in effect at December 31, 2020 is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ Operating ​ Year Ending December 31, Leases ​ 2021 ​ $ 28,203 ​ 2022 ​ ​ 26,454 ​ 2023 ​ ​ 25,335 ​ 2024 ​ ​ 24,938 ​ 2025 ​ ​ 24,163 ​ Thereafter ​ ​ 104,082 ​ Total lease payments ​ $ 233,175 ​ Less imputed interest ​ ​ (43,771) ​ Total ​ $ 189,404 ​

Lease Revenue (Tables)

Lease Revenue (Tables)	12 Months Ended
	Dec. 31, 2020
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, ​ ​ ​ 2020 ​ 2019 ​ 2018 ​ Lease revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum lease revenue ​ $ 432,781 ​ $ 409,622 ​ $ 390,691 ​ Variable lease revenue ​ ​ 89,970 ​ ​ 87,354 ​ ​ 83,992 ​ Total lease revenue ​ $ 522,751 ​ $ 496,976 ​ $ 474,683 ​
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, 2020, are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ Minimum ​ Year Ending December 31, Lease Payments ​ 2021 ​ $ 381,095 ​ 2022 ​ ​ 267,866 ​ 2023 ​ ​ 177,704 ​ 2024 ​ ​ 127,030 ​ 2025 ​ ​ 102,744 ​ Thereafter ​ ​ 199,977 ​ Total ​ $ 1,256,416 ​

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Debt
Summary of outstanding indebtedness	A summary of outstanding indebtedness as of December 31, 2020, and 2019, is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maturity ​ December 31, ​ December 31, ​ ​ Interest Rate Date 2020 2019 Revolving credit facility (1) ​ 1.39% and 3.01% at December 31, 2020, and December 31, 2019, respectively ​ November 8, 2023 ​ $ 148,500 ​ $ 62,500 ​ 2022 Senior unsecured term loan (2)(3) ​ 1.76% and 2.96% at December 31, 2020, and December 31, 2019, respectively ​ April 19, 2022 ​ ​ 200,000 ​ ​ 200,000 ​ 2023 Senior unsecured notes ​ 4.19% at December 31, 2020, and December 31, 2019, respectively ​ June 15, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured term loan (2)(3) ​ 2.86% and 3.44% at December 31, 2020, and December 31, 2019, respectively ​ April 19, 2024 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured notes ​ 3.91% at December 31, 2020, and December 31, 2019, respectively ​ April 20, 2024 ​ ​ 175,000 ​ ​ 175,000 ​ 2025 Senior unsecured term loan (2)(3) ​ 2.32% and 2.81% at December 31, 2020, and December 31, 2019, respectively ​ April 1, 2025 ​ ​ 350,000 ​ ​ 350,000 ​ 2026 Senior unsecured notes (2) ​ 4.52% at December 31, 2020, and December 31, 2019, respectively ​ April 17, 2026 ​ ​ 200,000 ​ ​ 200,000 ​ 2027 Senior unsecured notes ​ 3.75% at December 31, 2020 ​ May 6, 2027 ​ ​ 150,000 ​ ​ — ​ 2029 Senior unsecured notes ​ 4.31% at December 31, 2020, and December 31, 2019, respectively ​ April 17, 2029 ​ ​ 200,000 ​ ​ 200,000 ​ Total principal outstanding ​ ​ ​ ` ​ ​ 1,723,500 ​ ​ 1,487,500 ​ Unamortized deferred financing costs ​ ​ ​ ​ ​ ​ (7,589) ​ ​ (9,098) ​ Total debt ​ ​ ​ ​ ​ $ 1,715,911 ​ $ 1,478,402 ​ (1) Borrowings under the revolving credit facility bear interest at a variable rate per annum equal to either (i) LIBOR plus 125 basis points to 185 basis points, or (ii) a base rate plus 25 basis points to 85 basis points, each depending on our Operating Partnership’s leverage ratio. At December 31, 2020, our Operating Partnership’s leverage ratio was 33.6% and the interest rate was LIBOR plus 125 basis points. ​ (2) 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, 2020, and 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. ​ (3) Borrowings under the senior unsecured term loans 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, 2020, our Operating Partnership’s leverage ratio was 33.6% and the interest rate was LIBOR plus 120 basis points.
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, ​ ​ 2021 ​ $ — ​ 2022 ​ ​ 200,000 ​ 2023 ​ ​ 298,500 ​ 2024 ​ ​ 325,000 ​ 2025 ​ ​ 350,000 ​ Thereafter ​ ​ 550,000 ​ Total principal outstanding ​ ​ 1,723,500 ​ Unamortized deferred financing costs ​ ​ (7,589) ​ Total debt, net ​ $ 1,715,911 ​

Derivatives and Hedging Activ_2

Derivatives and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2020
Derivatives and Hedging Activities
Schedule of derivative instruments	The following table summarizes our derivative positions as of the years ended December 31, 2020, and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Notional Amount ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value (Level 2) (1) ​ December 31, ​ December 31, ​ Type of ​ ​ ​ ​ ​ ​ Effective ​ Expiration ​ December 31, ​ December 31, ​ 2020 ​ 2019 ​ Derivative ​ Index ​ Strike Rate ​ ​ Date ​ Date ​ 2020 ​ 2019 ​ $ — ​ $ 75,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 1.43 % ​ 5/5/2015 ​ 5/5/2020 (2) $ — ​ $ 67 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 2.72 ​ ​ 5/5/2018 ​ 4/5/2023 ​ ​ (4,388) ​ ​ (2,819) ​ ​ 100,000 ​ ​ 100,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 1.59 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ (5,657) ​ ​ 55 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 1.59 ​ ​ 11/8/2019 ​ 4/1/2025 ​ ​ (4,243) ​ ​ 41 ​ ​ 200,000 ​ ​ — ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 0.56 ​ ​ 3/5/2020 ​ 4/19/2022 ​ ​ (1,109) ​ ​ — ​ ​ 75,000 ​ ​ — ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 0.61 ​ ​ 3/5/2020 ​ 10/5/2023 ​ ​ (930) ​ ​ — ​ ​ 175,000 ​ ​ — ​ Interest Rate Swap ​ 1 mo. LIBOR ​ 0.64 ​ ​ 3/5/2020 ​ 10/1/2024 ​ ​ (2,773) ​ ​ — ​ $ 700,000 ​ $ 325,000 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ (19,100) ​ $ (2,656) ​ (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. ​ (2) On May 5, 2020, the $75 million five-year interest rate swap agreement, which reduced our variability in cash flows relating to interest payments based on one-month LIBOR variable debt, expired. ​
Schedule of derivatives effect on consolidated financial statements	The following table shows the effect of the derivatives on our consolidated financial statements (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ ​ 2020 ​ 2019 ​ 2018 ​ Cash Flow Hedges ​ ​ ​ ​ ​ ​ ​ ​ ​ Unrealized loss recognized in other comprehensive income $ (20,647) ​ $ (5,038) ​ $ (3,676) ​ (Gain) loss reclassified from other comprehensive income to interest expense ​ 5,016 ​ ​ 212 ​ ​ (326) ​ Interest expense ​ 45,086 ​ ​ 41,712 ​ ​ 35,526 ​

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2020
Stockholders' Equity
Schedule of dividends paid per share on Series A Cumulative Preferred Stock and common shares	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended December 31, ​ Record Date ​ 2020 ​ 2019 ​ 2018 ​ Common Stock: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Ordinary income (1) ​ $ 3.14 ​ $ 3.07 ​ $ 3.09 ​ Qualified dividend ​ ​ — ​ ​ — ​ ​ — ​ Capital gains ​ ​ — ​ ​ — ​ ​ — ​ Return of capital ​ ​ 1.74 ​ ​ 1.57 ​ ​ 0.93 ​ Total dividend (2) ​ $ 4.88 ​ $ 4.64 ​ $ 4.02 ​ (1) All taxable 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) Distributions made during calendar years ended December 31, 2020, 2019, and 2018 were subject to the tax treatment applicable to the year in which they are distributed.

Noncontrolling Interests - Op_2

Noncontrolling Interests - Operating Partnership (Tables)	12 Months Ended
	Dec. 31, 2020
Noncontrolling Interests - Operating Partnership
Schedule of ownership interests in our Operating Partnership	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ December 31, 2019 ​ ​ Number of Units Percentage of Total ​ Number of Units Percentage of Total ​ CoreSite Realty Corporation ​ 42,276,221 ​ 87.7 % 37,244,987 ​ 77.6 % Noncontrolling interests ​ 5,916,785 ​ 12.3 ​ 10,771,658 ​ 22.4 ​ Total ​ 48,193,006 ​ 100.0 % 48,016,645 ​ 100.0 %

Equity Incentive Plan (Tables)

Equity Incentive Plan (Tables)	12 Months Ended
	Dec. 31, 2020
Equity Incentive Plan
Schedule of stock option activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted ​ Weighted ​ Aggregate 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 ​ Granted ​ — ​ ​ — ​ ​ ​ ​ ​ Exercised ​ (26,164) ​ ​ 19.53 ​ ​ ​ $ 2.7 million ​ Forfeited ​ — ​ ​ — ​ ​ ​ ​ ​ Expired ​ — ​ ​ — ​ ​ ​ ​ ​ Options outstanding, December 31, 2020 ​ 5,582 ​ $ 19.76 ​ 0.7 years ​ $ 0.6 million ​
Schedule of restricted awards and RSUs	​ ​ ​ ​ ​ ​ ​ ​ ​ Restricted Stock Weighted 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 ​ Granted ​ 159,807 ​ ​ 109.81 ​ Forfeited ​ (23,980) ​ ​ 102.81 ​ Vested ​ (125,405) ​ ​ 90.77 ​ Unvested balance, December 31, 2020 ​ 302,795 ​ $ 105.35 ​
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, 2019 ​ 34,624 ​ 171,351 ​ 102,989 ​ $ 107.84 ​ Granted ​ 10,210 ​ 71,488 ​ 40,852 ​ ​ 125.02 ​ Performance adjustment (1) ​ 38,047 ​ (7,904) ​ 15,073 ​ ​ — ​ Forfeited ​ (1,049) ​ (4,878) ​ (2,964) ​ ​ 107.63 ​ Vested ​ (32,524) ​ (32,524) ​ (32,524) ​ ​ 105.55 ​ Unvested balance, December 31, 2020 ​ 49,308 ​ 197,533 ​ 123,426 ​ $ 113.96 ​ (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	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 2020 2019 2018 Expected term (in years) ​ 2.82 ​ ​ 2.82 ​ 2.82 ​ Expected volatility ​ 24.00 % ​ 24.09 % 24.15 % Expected annual dividend (1) ​ — ​ ​ — ​ — ​ Risk-free rate ​ 0.56 % ​ 2.48 % 2.24 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2020
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, ​ ​ 2020 2019 2018 Net income attributable to common shares ​ $ 79,309 ​ $ 75,840 ​ $ 77,922 ​ Weighted-average common shares outstanding - basic ​ ​ 40,453,438 ​ ​ 36,766,310 ​ ​ 34,957,244 ​ Effect of potentially dilutive common shares: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options ​ ​ 16,568 ​ ​ 36,685 ​ ​ 47,338 ​ Unvested awards ​ ​ 164,318 ​ ​ 140,878 ​ ​ 132,913 ​ Weighted-average common shares outstanding - diluted ​ ​ 40,634,324 ​ ​ 36,943,873 ​ ​ 35,137,495 ​ Net income per share attributable to common shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ $ 1.96 ​ $ 2.06 ​ $ 2.23 ​ Diluted ​ $ 1.95 ​ $ 2.05 ​ $ 2.22 ​

Quarterly Financial Informati_2

Quarterly Financial Information (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2020
Quarterly Financial Information (unaudited)
Schedule of selected quarterly information	The table below reflects the selected quarterly information for the years ended December 31, 2020, and 2019 (in thousands except share data): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ ​ December 31, ​ September 30, ​ June 30, ​ March 31, ​ ​ 2020 2020 2020 2020 Revenue ​ $ 154,938 ​ $ 153,981 ​ $ 150,543 ​ $ 147,362 ​ Operating income ​ ​ 34,352 ​ ​ 35,526 ​ ​ 35,693 ​ ​ 34,188 ​ Net income ​ ​ 22,409 ​ ​ 24,132 ​ ​ 25,088 ​ ​ 22,988 ​ Net income attributable to common shares ​ ​ 19,658 ​ ​ 21,132 ​ ​ 20,671 ​ ​ 17,848 ​ Net income per share attributable to common shares - basic ​ $ 0.46 ​ $ 0.50 ​ $ 0.52 ​ $ 0.48 ​ Net income per share attributable to common shares - diluted ​ $ 0.46 ​ $ 0.50 ​ $ 0.52 ​ $ 0.48 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​

Organization and Description _2

Organization and Description of Business (Details) - Operating partnership	Dec. 31, 2020	Dec. 31, 2019
Coresite Realty Corporation
Organization
Percentage of ownership interests held by the entity	87.70%	77.60%
Noncontrolling interests
Organization
Percentage of ownership interests held by others	12.30%	22.40%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Real Estate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investments in Real Estate
Capitalized interest costs	$ 12,800	$ 13,600	$ 5,600
Depreciation expense	$ 153,200	$ 136,000	$ 122,900
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, 2020	Dec. 31, 2019	Dec. 31, 2018
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	13,200,000	14,200,000	15,600,000
Internal sales commissions	16,453,000	15,064,000
Third party commissions	13,316,000	10,845,000
Other	357,000	462,000
Total	30,126,000	26,371,000
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, 2020	Dec. 31, 2019	Dec. 31, 2018
Internal-Use Software
Recognized impairment of internal use software	$ 0	$ 0	$ 0
Operating revenues:
Allowance for doubtful accounts (in dollars)	511	371
Asset Retirement and Environmental Remediation Obligations
Asset retirement and environmental remediation obligations	$ 1,800	$ 1,700
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, 2020item
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	Dec. 31, 2020ft²
Acquisitions
Net rentable space	200,000
Area of property (sqft)				54,798
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, 2020USD ($)	Dec. 31, 2019USD ($)	Apr. 12, 2019ft²
Investment in Real Estate
Land	$ 104,734	$ 94,593
Buildings and Improvements	2,273,536	1,989,731
Construction in Progress	319,411	394,474
Total investments in real estate	2,697,681	2,478,798
Net rentable space (in square feet) | ft²			200,000
Boston
Investment in Real Estate
Land	5,154	5,154
Buildings and Improvements	131,037	119,227
Construction in Progress	4,221	931
Total investments in real estate	140,412	125,312
Chicago
Investment in Real Estate
Land	7,059	5,493
Buildings and Improvements	183,282	115,699
Construction in Progress	58,883	100,118
Total investments in real estate	249,224	221,310
Denver
Investment in Real Estate
Buildings and Improvements	35,527	32,659
Construction in Progress	955	2,461
Total investments in real estate	36,482	35,120
Los Angeles
Investment in Real Estate
Land	22,127	18,672
Buildings and Improvements	451,999	376,525
Construction in Progress	68,052	60,178
Total investments in real estate	542,178	455,375
Miami
Investment in Real Estate
Land	728	728
Buildings and Improvements	14,679	14,491
Construction in Progress	10	133
Total investments in real estate	15,417	15,352
New York
Investment in Real Estate
Land	3,574	2,729
Buildings and Improvements	213,051	155,746
Construction in Progress	39,228	56,271
Total investments in real estate	255,853	214,746
Northern Virginia
Investment in Real Estate
Land	21,856	21,856
Buildings and Improvements	408,534	398,742
Construction in Progress	106,318	101,619
Total investments in real estate	536,708	522,217
San Francisco Bay
Investment in Real Estate
Land	44,236	39,961
Buildings and Improvements	835,427	776,642
Construction in Progress	41,744	72,763
Total investments in real estate	$ 921,407	$ 889,366

Other Assets (Details)

Other Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Other Assets.
Deferred rent receivable	$ 40,115	$ 38,335
Initial direct costs	30,126	26,371
Internal-use software	13,403	16,747
Prepaid expenses	11,572	7,675
Corporate furniture, fixtures and equipment	4,813	4,848
Deferred financing costs - revolving credit facility	2,382	3,148
Other	683	3,958
Total	103,094	$ 101,082
Estimated amortization of deferred leasing costs
2021	11,819
2022	7,835
2023	4,084
2024	1,890
2025	1,438
Thereafter	3,060
Total	$ 30,126

Leases (Details)

Leases (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)ft²	Dec. 31, 2019USD ($)ft²	Dec. 31, 2018USD ($)
Lease terms
Option to extend	P20Y
Weighted-average remaining lease term	9 years
Weighted average discount rate - Operating leases	4.90%
Increase in Right-of-Use Asset	$ 19,744	$ 1,040	$ 114,989
Lease expense
Operating lease expense	27,982	26,566	22,927
Variable lease expense	8,565	6,058	4,769
Rent expense	$ 36,547	$ 32,624	$ 27,696
Minimum
Lease terms
Remaining lease terms	1 year
Maximum
Lease terms
Remaining lease terms	15 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
DC1 Washington DC
Lease terms
Net Leased Space | ft²	25,000
Increase in Right-of-Use Asset	$ 11,800
Increase in operating lease liability	$ 11,800
DC2 Washington DC
Lease terms
Net Leased Space | ft²	25,000
Increase in Right-of-Use Asset	$ 7,000
Increase in operating lease liability	$ 7,000

Leases - Future Maturity of Lea

Leases - Future Maturity of Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2021	$ 28,203
2022	26,454
2023	25,335
2024	24,938
2025	24,163
Thereafter	104,082
Total lease payments	233,175
Less imputed interest	(43,771)
Total	$ 173,928	$ 172,976

Lease Revenue (Details)

Lease Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of lease revenue
Minimum lease revenue	$ 432,781	$ 409,622	$ 390,691
Variable lease revenue	89,970	87,354	83,992
Total lease revenue	522,751	$ 496,976	$ 474,683
Future minimum lease payments to be received under noncancelable leases
2021	381,095
2022	267,866
2023	177,704
2024	127,030
2025	102,744
Thereafter	199,977
Total	$ 1,256,416

Debt - Summary of Debt (Details

Debt - Summary of Debt (Details) - USD ($) $ in Thousands	May 05, 2020	Dec. 31, 2020	Jul. 14, 2020	May 06, 2020	Dec. 31, 2019	Nov. 08, 2019	Apr. 20, 2017	Apr. 19, 2017
Debt
Total principal outstanding		$ 1,723,500			$ 1,487,500
Unamortized deferred financing costs		(7,589)			(9,098)
Total debt		$ 1,715,911			$ 1,478,402
Term	5 years
Revolving Credit Facility
Debt
Interest rate as of reporting date (as a percent)		1.39%			3.01%
Total principal outstanding		$ 148,500			$ 62,500
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)		1.76%			2.96%
Total principal outstanding		$ 200,000			$ 200,000
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 Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)		2.86%			3.44%
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
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)		2.32%			2.81%
Total principal outstanding		$ 350,000			$ 350,000
2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)		4.52%			4.52%
Total principal outstanding		$ 200,000			$ 200,000
2027 Senior Unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)		3.75%
Total principal outstanding		$ 150,000
2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)		4.31%			4.31%
Total principal outstanding		$ 200,000			$ 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		$ 300,000						$ 200,000
Operating partnership | 2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding						150,000
Operating partnership | 2024 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding							$ 175,000
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%
Operating partnership | 2027 Senior Unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 50,000	$ 100,000

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility (Details) - USD ($) $ in Thousands	Nov. 08, 2019	Dec. 31, 2020	Dec. 31, 2019	Apr. 19, 2017
Debt
Total principal outstanding		$ 1,723,500	$ 1,487,500
Revolving Credit Facility
Debt
Total principal outstanding		148,500	62,500
Maximum borrowing capacity	$ 450,000	$ 450,000
Interest rate margin (as a percent)		125.00%
Debt Instrument, Basis Spread on Variable Rate		125.00%
Revolving Credit Facility | LIBOR | Minimum
Debt
Interest rate margin (as a percent)		125.00%
Debt Instrument, Basis Spread on Variable Rate		125.00%
Revolving Credit Facility | LIBOR | Maximum
Debt
Interest rate margin (as a percent)		185.00%
Debt Instrument, Basis Spread on Variable Rate		185.00%
Revolving Credit Facility | Base Rate | Minimum
Debt
Interest rate margin (as a percent)		25.00%
Debt Instrument, Basis Spread on Variable Rate		25.00%
Revolving Credit Facility | Base Rate | Maximum
Debt
Interest rate margin (as a percent)		85.00%
Debt Instrument, Basis Spread on Variable Rate		85.00%
Credit Agreement
Debt
Letters of credit outstanding		$ 6,100
Leverage ratio (as a percent)		33.60%
Secured debt ratio, Covenant (as a percent)		0.00%
Fixed charge coverage ratio		6
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%
Credit Agreement | Revolving Credit Facility | LIBOR
Debt
Interest rate margin (as a percent)		125.00%
Debt Instrument, Basis Spread on Variable Rate		125.00%
2022 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)		33.60%
2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)		180.00%
Debt Instrument, Basis Spread on Variable Rate		180.00%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		$ 200,000	200,000
2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		$ 150,000	150,000
Unsecured term loan	150,000
2025 Senior Unsecured Term Loan
Debt
Interest rate margin (as a percent)		120.00%
Debt Instrument, Basis Spread on Variable Rate		120.00%
2025 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)		180.00%
Debt Instrument, Basis Spread on Variable Rate		180.00%
2025 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)		120.00%
Debt Instrument, Basis Spread on Variable Rate		120.00%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		$ 350,000	$ 350,000
Leverage ratio (as a percent)		33.60%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)		20.00%
Debt Instrument, Basis Spread on Variable Rate		20.00%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)		80.00%
Debt Instrument, Basis Spread on Variable Rate		80.00%
Operating partnership | Base Rate | Minimum
Debt
Interest rate margin (as a percent)		20.00%
Debt Instrument, Basis Spread on Variable Rate		20.00%
Operating partnership | Credit Agreement
Debt
Extension fee (as a percent)		10.00%
Borrowing capacity at end of period		$ 450,000
Amount available for borrowing under the facility		$ 295,400
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)		120.00%
Debt Instrument, Basis Spread on Variable Rate		120.00%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)		120.00%
Debt Instrument, Basis Spread on Variable Rate		120.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)		80.00%
Debt Instrument, Basis Spread on Variable Rate		80.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding		$ 300,000		$ 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
Unsecured term loan	$ 350,000
Leverage ratio (as a percent)	33.60%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)	120.00%
Debt Instrument, Basis Spread on Variable Rate	120.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	120.00%
Debt Instrument, Basis Spread on Variable Rate	120.00%
Operating partnership | 2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	180.00%
Debt Instrument, Basis Spread on Variable Rate	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%
Debt Instrument, Basis Spread on Variable Rate	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 Instrument, Basis Spread on Variable Rate	80.00%

Debt - Unsecured Debt (Details)

Debt - Unsecured Debt (Details) - USD ($) $ in Thousands	May 06, 2020	Nov. 08, 2019	Dec. 31, 2020	Jul. 14, 2020	Dec. 31, 2019	Jul. 17, 2019	Apr. 17, 2019	Apr. 20, 2017	Apr. 19, 2017	Jun. 25, 2016
Debt
Total principal outstanding			$ 1,723,500		$ 1,487,500
2022 Senior Unsecured Term Loan
Debt
Leverage ratio (as a percent)			33.60%
2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)			180.00%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 200,000		$ 200,000
Effective interest rate			1.76%		2.96%
2024 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 150,000		$ 150,000
Effective interest rate			2.86%		3.44%
Unsecured term loan		$ 150,000
2025 Senior Unsecured Term Loan
Debt
Interest rate margin (as a percent)			120.00%
2025 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)			180.00%
2025 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)			120.00%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 350,000		$ 350,000
Effective interest rate			2.32%		2.81%
Leverage ratio (as a percent)			33.60%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)			20.00%
2025 Senior Unsecured Term Loan | Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)			80.00%
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		$ 200,000
Effective interest rate			4.52%		4.52%
2027 Senior Unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 150,000
Effective interest rate			3.75%
2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding			$ 200,000		$ 200,000
Effective interest rate			4.31%		4.31%
Operating partnership | Base Rate | Minimum
Debt
Interest rate margin (as a percent)			20.00%
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)			120.00%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)			120.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)			80.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding			$ 300,000						$ 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
Unsecured term loan		$ 350,000
Leverage ratio (as a percent)		33.60%
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 | Senior Unsecured Notes
Debt
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 | 2023 Senior unsecured notes | Senior Unsecured Notes | Private Placement
Debt
Total principal outstanding										$ 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
Total principal outstanding								$ 175,000
Stated interest rate								3.91%
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Effective interest rate			4.52%
Stated interest rate							4.11%
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 | Private Placement
Debt
Unsecured term loan							$ 200,000
Operating partnership | 2027 Senior Unsecured notes | Private Placement
Debt
Debt Instrument, Face Amount	$ 150,000
Effective interest rate	3.75%
Aggregate principal amount	$ 150,000
Operating partnership | 2027 Senior Unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding	$ 100,000			$ 50,000
Percentage of principal	100.00%
Percentage of notes purchased in cash	100.00%
Percentage of purchase price	100.00%
Operating partnership | 2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Debt Instrument, Face Amount							125,000
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, 2020	Dec. 31, 2019
Debt maturities
2022	$ 200,000
2023	298,500
2024	325,000
2025	350,000
Thereafter	550,000
Total principal outstanding	1,723,500	$ 1,487,500
Unamortized deferred financing costs	(7,589)	(9,098)
Total debt	$ 1,715,911	$ 1,478,402

Derivatives and Hedging Activ_3

Derivatives and Hedging Activities (Details) - USD ($) $ in Thousands	May 05, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative instruments and hedging activities disclosures
Notional Amount	$ 75,000
Derivative, Term of Contract	5 years
Term	5 years
Unrealized (loss) recorded in other comprehensive income			$ (20,647)	$ (5,038)	$ (3,676)
(Gain) loss reclassified from other comprehensive incomes to interest expense			(5,016)	(212)	326
Interest expense			45,086	41,712	35,526
Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			700,000	325,000
Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			(19,100)	(2,656)
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivative instruments and hedging activities disclosures
Unrealized (loss) recorded in other comprehensive income			(20,647)	(5,038)	(3,676)
(Gain) loss reclassified from other comprehensive incomes to interest expense			5,016	212	(326)
Interest expense			$ 45,086	41,712	$ 35,526
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		$ 7,500
Interest rate swap expiring 5/5/2020 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount				75,000
Strike Rate			1.43%
Interest rate swap expiring 5/5/2020 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value				67
Interest rate swap expiring 4/5/2023 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			$ 75,000	75,000
Strike Rate			2.72%
Interest rate swap expiring 4/5/2023 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (4,388)	(2,819)
Interest rate swap expiring 4/1/2025 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			$ 100,000	100,000
Strike Rate			1.59%
Interest rate swap expiring 4/1/2025 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (5,657)	55
Second interest rate swap expiring 4/1/2025 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			$ 75,000	75,000
Strike Rate			1.59%
Second interest rate swap expiring 4/1/2025 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (4,243)	$ 41
Interest rate swap expiring 4/19/2022 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			$ 200,000
Strike Rate			0.56%
Interest rate swap expiring 4/19/2022 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (1,109)
Interest rate swap expiring 10/5/2023 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			$ 75,000
Strike Rate			0.61%
Interest rate swap expiring 10/5/2023 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (930)
Interest rate swap expiring 10/12024 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount			$ 175,000
Strike Rate			0.64%
Interest rate swap expiring 10/12024 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value			$ (2,773)

Stockholders' Equity (Details)

Stockholders' Equity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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	42,768,153	37,701,042
Common Stock, shares outstanding	42,768,153	37,701,042
Proceeds from exercise of stock options	$ 483,000	$ 364,000	$ 310,000
2010 Plan
Stockholders' Equity
Proceeds from exercise of stock options	500,000	400,000	300,000
Amount of shares of common stock issued under the plan	$ 212,238	$ 188,741	$ 206,808
Operating partnership
Stockholders' Equity
Number of Operating Partnership Units (in shares)	48,193,006	48,016,645
Common Operating Partnership units redeemed for common stock (in shares)	4,701	3,610	11,068
Carlyle Group | Operating partnership
Stockholders' Equity
Common Operating Partnership units redeemed for common stock (in shares)	4,850,172	800,000	2,250,000

Stockholders' Equity - Dividend

Stockholders' Equity - Dividends (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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)	$ 3.14	$ 3.07	$ 3.09
Return of capital	1.74	1.57	0.93
Total dividend (in dollars per share)	$ 4.88	$ 4.64	$ 4.02

Noncontrolling Interests - Op_3

Noncontrolling Interests - Operating Partnership (Details) $ / shares in Units, $ in Thousands	Dec. 31, 2018$ / shares	Sep. 28, 2018$ / shares	Jun. 29, 2018$ / shares	Mar. 29, 2018$ / shares	Dec. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2019USD ($)shares	Dec. 31, 2018USD ($)shares
Noncontrolling Interests - Operating Partnership
Shares of common stock issued related to employee compensation arrangements					176,361
Closing price of the Company's stock (in dollars per share) | $ / shares					$ 125.28
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 | $					$ 22,676	$ 5,248	$ 21,002
Noncontrolling Interests | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					4,854,873	803,610	2,261,068
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $					$ 22,700	$ 5,200	$ 21,000
Operating partnership
Noncontrolling Interests - Operating Partnership
Conversion ratio for common operating partnership units					1
Number of Operating Partnership Units (in shares)					48,193,006	48,016,645
Percentage of Total					100.00%	100.00%
Quarterly distributions will receive by holders of operating partnership units (in dollars per unit) | $ / shares	$ 1.23	$ 1.22	$ 1.22	$ 1.22
Common Operating Partnership units redeemed for common stock (in shares)					4,701	3,610	11,068
Operating partnership | Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)					4,850,172	800,000	2,250,000
Operating partnership | Coresite Realty Corporation
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)					42,276,221	37,244,987
Percentage of Total					87.70%	77.60%
Operating partnership | Noncontrolling interests
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)					5,916,785	10,771,658
Percentage of Total					12.30%	22.40%
Operating partnership | Noncontrolling Interests
Noncontrolling Interests - Operating Partnership
Redemption value of the noncontrolling interest | $					$ 741,300,000

Equity Incentive Plan (Details)

Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity Incentive Plan
Shares of common stock reserved for issuance	6,000,000
Shares of common stock that were available for issuance	2,480,212
Stock Options
Number of Shares Subject to Option
Options outstanding at the beginning of the period (in shares)	31,746	49,133	67,789
Exercised (in shares)	(26,164)	(17,387)	(18,656)
Options outstanding at the end of the period (in shares)	5,582	31,746	49,133	67,789
Weighted Average Remaining Contractual Term
Options outstanding at the end of the period	8 months 12 days	1 year 4 months 24 days	2 years 7 months 6 days	3 years 4 months 24 days
Aggregate Intrinsic Value
Outstanding at the end of the period	$ 0.6	$ 2.9	$ 3.3	$ 6.4
Exercised	$ 2.7	$ 1.7	$ 1.6
Stock Options | Weighted Average
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share)	$ 19.57	$ 20.06	$ 19.12
Exercised (in dollars per share)	19.53	20.94	16.63
Options outstanding at the end of the period (in dollars per share)	$ 19.76	$ 19.57	$ 20.06	$ 19.12
Restricted Awards
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	292,373	284,879	289,843
Granted (in shares)	159,807	160,361	155,697
Forfeited (in shares)	(23,980)	(21,910)	(36,546)
Vested (in shares)	(125,405)	(130,957)	(124,115)
Unvested balance at the end of the period (in shares)	302,795	292,373	284,879	289,843
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share)	$ 96.44	$ 85.98	$ 70.71
Granted (in dollars per share)	109.81	99.91	96.47
Forfeited (in dollars per share)	102.81	94.03	84.10
Vested (in dollars per share)	90.77	78.32	64.03
Unvested balance at the end of the period (in dollars per share)	$ 105.35	$ 96.44	$ 85.98	$ 70.71
Compensation cost not yet recognized
Unearned compensation	$ 21.5
Vesting period	2 years 4 months 24 days
PSAs
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	107.84
Granted (in shares)	125.02
Unvested balance at the end of the period (in shares)		107.84
Weighted Average Fair Value at Grant
Forfeited (in dollars per share)	$ 107.63
Vested (in dollars per share)	105.55
Unvested balance at the end of the period (in dollars per share)	$ 113.96
Compensation cost not yet recognized
Unearned compensation	$ 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 25 days
Expected volatility (as a percent)	24.00%	24.09%	24.15%
Risk-free rate (as a percent)	0.56%	2.48%	2.24%
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)	34,624
Granted (in shares)	10,210
Performance adjustment	38,047
Forfeited (in shares)	(1,049)
Vested (in shares)	(32,524)
Unvested balance at the end of the period (in shares)	49,308	34,624
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)	171,351
Granted (in shares)	71,488
Performance adjustment	(7,904)
Forfeited (in shares)	(4,878)
Vested (in shares)	(32,524)
Unvested balance at the end of the period (in shares)	197,533	171,351
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)	102,989
Granted (in shares)	40,852
Performance adjustment	15,073
Forfeited (in shares)	(2,964)
Vested (in shares)	(32,524)
Unvested balance at the end of the period (in shares)	123,426	102,989

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share
Net income attributable to common shares	$ 19,658	$ 21,132	$ 20,671	$ 17,848	$ 19,194	$ 17,450	$ 19,535	$ 19,661	$ 79,309	$ 75,840	$ 77,922
Weighted-average common shares outstanding - basic									40,453,438	36,766,310	34,957,244
Effect of potentially dilutive common shares:
Weighted-average common shares outstanding - diluted									40,634,324	36,943,873	35,137,495
Net income per share attributable to common shares
Basic	$ 0.46	$ 0.50	$ 0.52	$ 0.48	$ 0.51	$ 0.47	$ 0.54	$ 0.54	$ 1.96	$ 2.06	$ 2.23
Diluted	$ 0.46	$ 0.50	$ 0.52	$ 0.48	$ 0.51	$ 0.47	$ 0.53	$ 0.54	$ 1.95	$ 2.05	$ 2.22
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share									3,933	2,546	132,749
Stock Options
Effect of potentially dilutive common shares:
Potentially dilutive common shares									16,568	36,685	47,338
Restricted awards
Effect of potentially dilutive common shares:
Potentially dilutive common shares									164,318	140,878	132,913

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Thousands	Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
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		$ 1,900	$ 1,700	$ 1,200

Estimated Fair Value of Finan_2

Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Estimated Fair Value of Financial Instruments
Total principal outstanding	$ 1,723,500	$ 1,487,500

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Telecommunications Capacity
Commitments and Contingencies
Open commitments		$ 71
Telecommunications Capacity | Forecast
Commitments and Contingencies
Open commitments met	$ 7.8
Construction Contracts
Commitments and Contingencies
Open commitments		$ 39.9

Quarterly Financial Informati_3

Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information (unaudited)
Revenues	$ 154,938	$ 153,981	$ 150,543	$ 147,362	$ 146,035	$ 144,891	$ 142,906	$ 138,895	$ 606,824	$ 572,727	$ 544,392
Operating income	34,352	35,526	35,693	34,188	35,684	33,643	36,056	35,433	139,759	140,816	142,279
Net income	22,409	24,132	25,088	22,988	24,745	22,644	25,743	25,905	94,617	99,037	106,763
Net income attributable to common shares	$ 19,658	$ 21,132	$ 20,671	$ 17,848	$ 19,194	$ 17,450	$ 19,535	$ 19,661	$ 79,309	$ 75,840	$ 77,922
Net income per share attributable to common shares - basic (in dollars per share)	$ 0.46	$ 0.50	$ 0.52	$ 0.48	$ 0.51	$ 0.47	$ 0.54	$ 0.54	$ 1.96	$ 2.06	$ 2.23
Net income per share attributable to common shares - diluted (in dollars per share)	$ 0.46	$ 0.50	$ 0.52	$ 0.48	$ 0.51	$ 0.47	$ 0.53	$ 0.54	$ 1.95	$ 2.05	$ 2.22

Schedule III Real Estate and _2

Schedule III Real Estate and Accumulated Depreciation - Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Initial Cost
Land	$ 146,197
Building and Improvements	428,403
Costs Capitalized Subsequent to Acquisition	2,123,081
Gross Amount Carried As of Year End
Land	104,734
Building and Improvements	2,592,947
Total	2,697,681	$ 2,478,798	$ 2,083,205	$ 1,821,217
Accumulated Depreciation	867,975	$ 720,498	$ 590,784	$ 473,141
Aggregate cost of the total properties for federal income tax purposes	2,432,200
BO1 Somerville MA
Initial Cost
Land	6,100
Building and Improvements	26,748
Costs Capitalized Subsequent to Acquisition	107,564
Gross Amount Carried As of Year End
Land	5,154
Building and Improvements	135,258
Total	140,412
Accumulated Depreciation	73,967
CH1 Chicago IL
Initial Cost
Land	5,493
Building and Improvements	49,522
Costs Capitalized Subsequent to Acquisition	67,728
Gross Amount Carried As of Year End
Land	5,493
Building and Improvements	117,250
Total	122,743
Accumulated Depreciation	61,401
CH2 Chicago IL
Initial Cost
Land	4,383
Costs Capitalized Subsequent to Acquisition	122,098
Gross Amount Carried As of Year End
Land	1,566
Building and Improvements	124,915
Total	126,481
Accumulated Depreciation	2,432
DC1 Washington DC
Initial Cost
Building and Improvements	4,797
Costs Capitalized Subsequent to Acquisition	5,398
Gross Amount Carried As of Year End
Building and Improvements	10,195
Total	10,195
Accumulated Depreciation	8,412
DC2 Washington DC
Initial Cost
Costs Capitalized Subsequent to Acquisition	23,920
Gross Amount Carried As of Year End
Building and Improvements	23,920
Total	23,920
Accumulated Depreciation	4,648
DE1 Denver CO
Initial Cost
Building and Improvements	37
Costs Capitalized Subsequent to Acquisition	33,377
Gross Amount Carried As of Year End
Building and Improvements	33,414
Total	33,414
Accumulated Depreciation	13,065
DE2 Denver CO
Initial Cost
Building and Improvements	4
Costs Capitalized Subsequent to Acquisition	3,064
Gross Amount Carried As of Year End
Building and Improvements	3,068
Total	3,068
Accumulated Depreciation	1,314
LA1 Los Angeles CA [Member]
Initial Cost
Building and Improvements	41,657
Costs Capitalized Subsequent to Acquisition	64,257
Gross Amount Carried As of Year End
Building and Improvements	105,914
Total	105,914
Accumulated Depreciation	67,543
LA2 Los Angeles CA [Member]
Initial Cost
Land	18,672
Building and Improvements	94,114
Costs Capitalized Subsequent to Acquisition	181,631
Gross Amount Carried As of Year End
Land	18,672
Building and Improvements	275,745
Total	294,417
Accumulated Depreciation	115,786
LA3 Los Angeles CA
Initial Cost
Land	9,795
Costs Capitalized Subsequent to Acquisition	129,116
Gross Amount Carried As of Year End
Land	3,455
Building and Improvements	135,456
Total	138,911
Accumulated Depreciation	888
LA4 Los Angeles CA
Initial Cost
Building and Improvements	2,533
Costs Capitalized Subsequent to Acquisition	403
Gross Amount Carried As of Year End
Building and Improvements	2,936
Total	2,936
Accumulated Depreciation	1,630
MI1 Miami FL [Member]
Initial Cost
Land	728
Building and Improvements	9,325
Costs Capitalized Subsequent to Acquisition	5,364
Gross Amount Carried As of Year End
Land	728
Building and Improvements	14,689
Total	15,417
Accumulated Depreciation	5,841
NY1 New York NY [Member]
Initial Cost
Costs Capitalized Subsequent to Acquisition	46,274
Gross Amount Carried As of Year End
Building and Improvements	46,274
Total	46,274
Accumulated Depreciation	34,027
NY2 Secaucus NJ [Member]
Initial Cost
Land	4,952
Building and Improvements	18,408
Costs Capitalized Subsequent to Acquisition	186,219
Gross Amount Carried As of Year End
Land	3,574
Building and Improvements	206,005
Total	209,579
Accumulated Depreciation	47,551
SV1 San Jose CA [Member]
Initial Cost
Land	6,863
Building and Improvements	91,719
Costs Capitalized Subsequent to Acquisition	44,561
Gross Amount Carried As of Year End
Land	6,863
Building and Improvements	136,280
Total	143,143
Accumulated Depreciation	56,546
SV2 Milpitas CA [Member]
Initial Cost
Land	5,086
Building and Improvements	5,046
Costs Capitalized Subsequent to Acquisition	29,152
Gross Amount Carried As of Year End
Land	5,086
Building and Improvements	34,198
Total	39,284
Accumulated Depreciation	24,996
SV3 Santa Clara CA [Member]
Initial Cost
Land	4,162
Building and Improvements	3,482
Costs Capitalized Subsequent to Acquisition	51,217
Gross Amount Carried As of Year End
Land	3,972
Building and Improvements	54,889
Total	58,861
Accumulated Depreciation	47,541
SV4 Santa Clara CA [Member]
Initial Cost
Land	4,632
Building and Improvements	3,716
Costs Capitalized Subsequent to Acquisition	101,126
Gross Amount Carried As of Year End
Land	4,501
Building and Improvements	104,973
Total	109,474
Accumulated Depreciation	61,835
SV5 Santa Clara CA [Member]
Initial Cost
Land	2,572
Costs Capitalized Subsequent to Acquisition	23,653
Gross Amount Carried As of Year End
Land	2,544
Building and Improvements	23,681
Total	26,225
Accumulated Depreciation	4,502
SV6 Santa Clara CA
Initial Cost
Land	4,741
Costs Capitalized Subsequent to Acquisition	36,868
Gross Amount Carried As of Year End
Land	4,825
Building and Improvements	36,784
Total	41,609
Accumulated Depreciation	4,312
SV7 Santa Clara CA
Initial Cost
Land	3,793
Costs Capitalized Subsequent to Acquisition	248,523
Gross Amount Carried As of Year End
Land	3,595
Building and Improvements	248,721
Total	252,316
Accumulated Depreciation	59,213
SV8 Santa Clara CA
Initial Cost
Land	12,158
Costs Capitalized Subsequent to Acquisition	204,949
Gross Amount Carried As of Year End
Land	12,850
Building and Improvements	204,257
Total	217,107
Accumulated Depreciation	14,211
SV9 Santa Clara CA
Initial Cost
Land	25,767
Building and Improvements	407
Costs Capitalized Subsequent to Acquisition	7,214
Gross Amount Carried As of Year End
Building and Improvements	33,388
Total	33,388
Accumulated Depreciation	407
VA1 Reston VA [Member]
Initial Cost
Land	6,903
Building and Improvements	32,939
Costs Capitalized Subsequent to Acquisition	98,080
Gross Amount Carried As of Year End
Land	6,779
Building and Improvements	131,143
Total	137,922
Accumulated Depreciation	90,229
VA2 Reston VA [Member]
Initial Cost
Land	5,197
Costs Capitalized Subsequent to Acquisition	135,875
Gross Amount Carried As of Year End
Land	5,539
Building and Improvements	135,533
Total	141,072
Accumulated Depreciation	52,050
VA3 Reston VA
Initial Cost
Land	8,529
Building and Improvements	26,411
Costs Capitalized Subsequent to Acquisition	165,446
Gross Amount Carried As of Year End
Land	3,866
Building and Improvements	196,520
Total	200,386
Accumulated Depreciation	11,676
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	$ 1,951

Schedule III Real Estate and _3

Schedule III Real Estate and Accumulated Depreciation - Rollforwards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property
Balance, beginning of year	$ 2,478,798	$ 2,083,205	$ 1,821,217
Additions - property acquisitions		26,174	7,474
Additions - improvements	218,883	369,419	254,514
Balance, end of year	2,697,681	2,478,798	2,083,205
Accumulated Depreciation
Balance, beginning of year	720,498	590,784	473,141
Additions, net of disposals	147,477	129,714	117,643
Balance, end of year	$ 867,975	$ 720,498	$ 590,784