CoreSite Realty 10-Q 1 Nov 19 2019 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2019	Oct. 30, 2019
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Sep. 30, 2019
Entity File Number	001-34877
Entity Registrant Name	CoreSite Realty Corp
Entity Incorporation, State or Country Code	MD
Entity Tax Identification Number	27-1925611
Entity Address, Address Line One	1001 17th Street, Suite 500
Entity Address, City or Town	Denver
Entity Address, State or Province	CO
Entity Address, Postal Zip Code	80202
City Area Code	866
Local Phone Number	777-2673
Title of 12(b) Security	Common Stock, $0.01 par value per share
Trading Symbol	COR
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		37,687,013
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Entity Central Index Key	0001490892
Amendment Flag	false

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments in real estate:
Land	$ 90,300	$ 86,955
Buildings and improvements	1,896,820	1,730,329
Total investments in real estate	1,987,120	1,817,284
Less: Accumulated depreciation and amortization	(686,026)	(590,784)
Net investment in operating properties	1,301,094	1,226,500
Construction in progress	389,174	265,921
Net investments in real estate	1,690,268	1,492,421
Operating lease right-of-use assets, net	177,535	190,304
Cash and cash equivalents	4,703	2,599
Accounts and other receivables, net of allowance for doubtful accounts of $214 and $417 as of September 30, 2019, and December 31, 2018, respectively	26,330	18,464
Lease intangibles, net of accumulated amortization of $4,938 and $7,756 as of September 30, 2019, and December 31, 2018, respectively	4,328	6,943
Goodwill	40,646	40,646
Other assets, net	102,932	102,290
Total assets	2,046,742	1,853,667
Liabilities:
Debt, net of unamortized deferred financing costs of $6,709 and $7,756 as of September 30, 2019, and December 31, 2018, respectively	1,380,541	1,130,823
Operating lease liabilities	191,725	202,699
Accounts payable and accrued expenses	132,739	89,315
Accrued dividends and distributions	61,783	55,679
Acquired below-market lease contracts, net of accumulated amortization of $1,460 and $3,840 as of September 30, 2019, and December 31, 2018, respectively	2,562	2,846
Unearned revenue, prepaid rent and other liabilities	34,066	37,672
Total liabilities	1,803,416	1,519,034
Stockholders' equity:
Common Stock, par value $0.01, 100,000,000 shares authorized and 37,688,382 and 36,708,691 shares issued and outstanding at September 30, 2019, and December 31, 2018, respectively	373	363
Additional paid-in capital	508,209	491,314
Accumulated other comprehensive loss	(6,666)	(2,193)
Distributions in excess of net income	(321,720)	(246,929)
Total stockholders' equity	180,196	242,555
Noncontrolling interests	63,130	92,078
Total equity	243,326	334,633
Total liabilities and equity	$ 2,046,742	$ 1,853,667

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
CONDENSED CONSOLIDATED BALANCE SHEETS
Allowance for doubtful accounts (in dollars)	$ 214	$ 417
Accumulated amortization, lease intangibles (in dollars)	4,938	7,756
Unamortized Debt Issuance Expense	6,709	5,677
Accumulated amortization of below-market lease	$ 1,460	$ 3,840
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	100,000,000	100,000,000
Common Stock, shares issued	37,688,382	36,708,691
Common Stock, shares outstanding	37,688,382	36,708,691

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Data center revenue:
Interconnection revenue	$ 19,082	$ 17,701	$ 56,274	$ 51,683
Total operating revenues	144,891	139,180	426,692	405,246
Operating expenses:
Property operating and maintenance	41,251	41,161	117,428	112,870
Real estate taxes and insurance	4,973	4,699	17,157	14,329
Depreciation and amortization	40,546	36,264	113,188	105,598
Sales and marketing	5,476	5,180	16,912	15,629
General and administrative	10,671	10,074	33,123	29,556
Rent	8,331	7,329	23,752	20,276
Transaction costs				75
Total operating expenses	111,248	104,707	321,560	298,333
Operating income	33,643	34,473	105,132	106,913
Interest expense	(10,986)	(9,433)	(30,795)	(26,078)
Income before income taxes	22,657	25,040	74,337	80,835
Income tax (expense) benefit	(13)	(20)	(45)	30
Net income	22,644	25,020	74,292	80,865
Net income attributable to noncontrolling interests	5,194	6,420	17,646	22,574
Net income attributable to common shares	$ 17,450	$ 18,600	$ 56,646	$ 58,291
Net income per share attributable to common shares:
Basic	$ 0.47	$ 0.52	$ 1.55	$ 1.69
Diluted	$ 0.47	$ 0.52	$ 1.54	$ 1.68
Weighted average common shares outstanding
Basic	36,950,950	35,512,091	36,589,593	34,504,790
Diluted	37,132,155	35,721,478	36,763,241	34,693,835
Rental, power, and related revenue
Data center revenue:
Rental revenue	$ 122,598	$ 118,590	$ 361,534	$ 344,745
Office, light-industrial and other revenue
Data center revenue:
Rental revenue	$ 3,211	$ 2,889	$ 8,884	$ 8,818

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income	$ 22,644	$ 25,020	$ 74,292	$ 80,865
Other comprehensive (loss) income:
Unrealized (loss) gain on derivative contracts	(463)	523	(5,608)	1,356
Reclassification of other comprehensive income (loss) to interest expense	137	(69)	(44)	(145)
Comprehensive income	22,318	25,474	68,640	82,076
Comprehensive income attributable to noncontrolling interests	5,125	6,534	16,293	22,907
Comprehensive income attributable to common shares	$ 17,193	$ 18,940	$ 52,347	$ 59,169

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		70			70	(70)
Redemption of noncontrolling interests (in shares)	7,056
Issuance of stock awards, net of forfeitures (in shares)	201,181
Exercise of stock options		100			100		100
Exercise of stock options (in shares)	5,161
Share-based compensation	$ 2	2,739			2,741		2,741
Dividends and distributions				(33,749)	(33,749)	(13,553)	(47,302)
Net income				20,302	20,302	8,264	28,566
Other comprehensive income			410		410	167	577
Balance at Mar. 31, 2018	$ 340	460,404	1,163	(191,013)	270,894	132,432	403,326
Balance (in shares) at Mar. 31, 2018	34,454,213
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
Net income							80,865
Balance at Sep. 30, 2018	$ 363	487,848	1,758	(226,184)	263,785	99,840	363,625
Balance (in shares) at Sep. 30, 2018	36,698,420
Balance at Mar. 31, 2018	$ 340	460,404	1,163	(191,013)	270,894	132,432	403,326
Balance (in shares) at Mar. 31, 2018	34,454,213
Increase (Decrease) in Stockholders' Equity
Issuance of stock awards, net of forfeitures (in shares)	(2,205)
Exercise of stock options		119			119		119
Exercise of stock options (in shares)	7,720
Share-based compensation		3,364			3,364		3,364
Dividends and distributions				(35,424)	(35,424)	(14,244)	(49,668)
Net income				19,389	19,389	7,890	27,279
Other comprehensive income			128		128	52	180
Balance at Jun. 30, 2018	$ 340	463,887	1,291	(207,048)	258,470	126,130	384,600
Balance (in shares) at Jun. 30, 2018	34,459,728
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests	$ 23	20,747	127		20,897	(20,897)
Redemption of noncontrolling interests (in shares)	2,250,000
Issuance of stock awards, net of forfeitures (in shares)	(11,308)
Share-based compensation		3,214			3,214		3,214
Dividends and distributions				(37,736)	(37,736)	(11,927)	(49,663)
Net income				18,600	18,600	6,420	25,020
Other comprehensive income			340		340	114	454
Balance at Sep. 30, 2018	$ 363	487,848	1,758	(226,184)	263,785	99,840	363,625
Balance (in shares) at Sep. 30, 2018	36,698,420
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
Issuance of stock awards, net of forfeitures (in shares)	192,009
Exercise of stock options		17			17		17
Exercise of stock options (in shares)	1,129
Share-based compensation	$ 1	3,592			3,593		3,593
Dividends and distributions				(40,581)	(40,581)	(12,733)	(53,314)
Net income				19,661	19,661	6,244	25,905
Other comprehensive income			(3,540)		(3,540)	(1,124)	(4,664)
Balance at Mar. 31, 2019	$ 364	494,923	(5,733)	(267,849)	221,705	84,465	306,170
Balance (in shares) at Mar. 31, 2019	36,901,829
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
Net income							74,292
Balance at Sep. 30, 2019	$ 373	508,209	(6,666)	(321,720)	180,196	63,130	243,326
Balance (in shares) at Sep. 30, 2019	37,688,382
Balance at Mar. 31, 2019	$ 364	494,923	(5,733)	(267,849)	221,705	84,465	306,170
Balance (in shares) at Mar. 31, 2019	36,901,829
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests		20			20	(20)
Redemption of noncontrolling interests (in shares)	2,770
Issuance of stock awards, net of forfeitures (in shares)	(15,567)
Exercise of stock options		21			21		21
Exercise of stock options (in shares)	1,339
Share-based compensation		3,864			3,864		3,864
Dividends and distributions				(44,895)	(44,895)	(14,118)	(59,013)
Net income				19,535	19,535	6,208	25,743
Other comprehensive income			(502)		(502)	(160)	(662)
Balance at Jun. 30, 2019	$ 364	498,828	(6,235)	(293,209)	199,748	76,375	276,123
Balance (in shares) at Jun. 30, 2019	36,890,371
Increase (Decrease) in Stockholders' Equity
Redemption of noncontrolling interests	$ 8	5,394	(174)		5,228	(5,228)
Redemption of noncontrolling interests (in shares)	800,840
Issuance of stock awards, net of forfeitures (in shares)	(3,983)
Exercise of stock options		28			28		28
Exercise of stock options (in shares)	1,154
Share-based compensation	$ 1	3,959			3,960		3,960
Dividends and distributions				(45,961)	(45,961)	(13,142)	(59,103)
Net income				17,450	17,450	5,194	22,644
Other comprehensive income			(257)		(257)	(69)	(326)
Balance at Sep. 30, 2019	$ 373	$ 508,209	$ (6,666)	$ (321,720)	$ 180,196	$ 63,130	$ 243,326
Balance (in shares) at Sep. 30, 2019	37,688,382

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 74,292	$ 80,865
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	113,188	105,598
Amortization of above/below market leases	(219)	(494)
Amortization of deferred financing costs and hedge amortization	2,368	1,756
Share-based compensation	10,781	8,864
Bad debt expense	106	(300)
Changes in operating assets and liabilities:
Accounts receivable	(7,971)	5,001
Deferred rent receivable	4,218	(2,930)
Deferred leasing costs	(9,727)	(9,140)
Other assets	(8,395)	(5,342)
Accounts payable and accrued expenses	15,800	6,274
Unearned revenue, prepaid rent and other liabilities	(8,765)	(1,201)
Operating leases	2,035	(215)
Net cash provided by operating activities	187,711	188,736
CASH FLOWS FROM INVESTING ACTIVITIES
Tenant improvements	(3,167)	(4,191)
Real estate improvements	(239,526)	(160,269)
Business combinations and asset acquisitions	(26,060)	(10,681)
Net cash used in investing activities	(268,753)	(175,141)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercise of stock options	66	219
Proceeds from revolving credit facility	286,250	184,036
Payments on revolving credit facility	(435,500)	(199,036)
Proceeds from unsecured debt	400,000	150,000
Payments of loan fees and costs	(2,344)	(4,986)
Dividends and distributions	(165,326)	(143,769)
Net cash provided by (used in) financing activities	83,146	(13,536)
Net change in cash and cash equivalents	2,104	59
Cash and cash equivalents, beginning of period	2,599	5,247
Cash and cash equivalents, end of period	4,703	5,306
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest, net of capitalized amounts	21,686	21,209
Cash paid for operating lease liabilities	18,581	18,116
NON-CASH INVESTING AND FINANCING ACTIVITY
Construction costs payable capitalized to real estate	65,964	37,816
Accrual of dividends and distributions	61,783	51,840
NON-CASH OPERATING ACTIVITY
Lease liabilities arising from obtaining right-of-use assets	$ 1,035	$ 114,989

Organization and Description of

Organization and Description of Business	9 Months Ended
	Sep. 30, 2019
Organization and Description of Business
Organization and Description of Business	1. Organization and Description of Business ​ CoreSite Realty Corporation (the “Company,” “we,” “us,” or “our”) was organized in the State of Maryland on February 17, 2010, and is a fully-integrated, self-administered, and self-managed real estate investment trust (“REIT”). Through our controlling interest in CoreSite, L.P. (our “Operating Partnership”), we are engaged in the business of owning, acquiring, constructing and operating data centers. As of September 30, 2019, the Company owned a 77.6% common interest in our Operating Partnership, and affiliates of The Carlyle Group and others owned a 22.4% interest in our Operating Partnership. See additional discussion in Note 10, Noncontrolling Interests — Operating Partnership.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2019
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies ​ Principles of Consolidation and Basis of Presentation ​ The accompanying unaudited condensed consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. Accordingly, these unaudited condensed consolidated financial statements do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. The results of operations for the three and nine months ended September 30, 2019, are not necessarily indicative of the expected results for the year ending December 31, 2019, or any other future period. These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto included in our Annual Report on Form 10-K ​ 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 Not Yet Adopted ​ 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 We do not expect the provisions of ASU 2018-15 to have a material impact on our condensed consolidated financial statements. We determined that all other recently issued accounting pronouncements will not have a material impact on our consolidated financial statements or do not apply to our operations. Use of Estimates ​ The preparation of these unaudited condensed 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 unaudited condensed 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 $3.7 million and $1.3 million for the three months ended September 30, 2019, and 2018, respectively. Capitalized interest costs were $10.0 million and $3.5 million for the nine months ended September 30, 2019, and 2018, respectively. ​ Depreciation and amortization are calculated using the straight-line method over the following useful lives of the assets: ​ ​ ​ ​ Buildings 27 to 40 years Building improvements ​ 1 to 10 years Leasehold improvements ​ The shorter of the lease term or useful life of the asset ​ Depreciation expense was $35.1 million and $31.4 million for the three months ended September 30, 2019, and 2018, respectively. Depreciation expense was $100.1 million and $91.3 million for the nine months ended September 30, 2019, and 2018, respectively. ​ Acquisition of Investment in Real Estate ​ When accounting for business combinations and asset acquisitions, the fair value of the real estate acquired is allocated to the acquired tangible assets, consisting primarily of land, building and building improvements, and identified intangible assets and liabilities, consisting of the value of above-market and below-market leases, value of in-place leases and the value of customer relationships. The primary difference between business combinations and asset acquisitions is that asset acquisitions require cost accumulation and allocation at a relative fair value. Acquisition costs are capitalized for asset acquisitions and 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 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 three or nine months ended September 30, 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 September 30, 2019, and December 31, 2018, we had $40.6 million of goodwill at each date. The Company’s goodwill has an indeterminate life and is not amortized, but is tested for impairment on an annual basis, or more frequently if events or changes in circumstances indicate that the asset might be impaired. No impairment loss was recognized for the three or nine months ended September 30, 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 $4.0 million for both the three months ended September 30, 2019, and 2018. Amortization of initial direct costs was $10.8 million and $11.8 million for the nine months ended September 30, 2019, and 2018, respectively. Initial direct costs are included within other assets in the condensed consolidated balance sheets and consisted of the following, net of amortization, as of September 30, 2019, and December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2019 2018 Internal sales commissions ​ $ 15,340 ​ $ 14,199 ​ Third party commissions ​ ​ 11,066 ​ ​ 9,855 ​ Other ​ ​ 485 ​ ​ 597 ​ Total ​ $ 26,891 ​ $ 24,651 ​ ​ Deferred Financing Costs ​ Deferred financing costs include costs incurred in connection with obtaining debt and extending existing debt. These financing costs are capitalized and amortized on a straight-line basis, which approximates the effective-interest method, over the term of the indebtedness and the amortization is included as a component of interest expense. Depending on the type of debt instrument, deferred financing costs are reported either in other assets or as a direct deduction from the carrying amount of the related debt liabilities in our condensed consolidated balance sheets. ​ Recoverability of Long-Lived Assets ​ We review our long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Impairment is recognized when estimated expected future cash flows (undiscounted and without interest charges) are less than the carrying amount of the assets. The estimation of expected future net cash flows is inherently uncertain and relies, to a considerable extent, on assumptions regarding current and future economics and market conditions and the availability of capital. If, in future periods, there are changes in the estimates or assumptions incorporated into the impairment review analysis, the changes could result in an adjustment to the carrying amount of the long-lived assets. To the extent that impairment has occurred, the excess of the carrying amount of long-lived assets over its estimated fair value would be recognized as an impairment loss charged to net income. For the three and nine months ended September 30, 2019, and 2018, no impairment of long-lived assets was recognized in the condensed consolidated financial statements. ​ Derivative Instruments and Hedging Activities ​ We reflect all derivative instruments at fair value as either assets or liabilities on the condensed 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 8, 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. No impairment of internal-use software was recognized in the condensed consolidated financial statements for the three and nine months ended September 30, 2019, and 2018. ​ 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 condensed 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 condensed consolidated statements of operations. For fixed power arrangements, a customer pays us a fixed monthly fee for a committed available amount of power. We recognize the fixed power revenue each month over the term of the lease. For variable power arrangements, a customer pays us variable monthly fees for the specific amount of power utilized at the current utility rates. We recognize variable power revenue each month as the uncertainty related to the consideration is resolved, as power is provided to our customers, and as our customers utilize the power. ​ Some of our leases contain provisions under which our customers reimburse us for common area maintenance and other executory costs. These customer reimbursements are variable and are recognized in the period that the expenses are recognized. These services have the same pattern of transfer over the lease term and are also combined with lease revenue within our condensed consolidated statements of operations. ​ Interconnection Revenue ​ We also derive revenue from interconnection services, which are generally contracted on a month-to-month basis cancellable by us or the customer at any time. Interconnection services are accounted for as separate contracts and are not combined with lease and power arrangements. We recognize interconnection revenue each month as these services are delivered to, and utilized by, our customers. ​ Allowance for Doubtful Accounts ​ A provision for uncollectible accounts is recorded if a receivable balance relating to contractual rent, rental revenue recorded on a straight-line basis, tenant reimbursements or other billed amounts is considered by management to be uncollectible. At September 30, 2019, and December 31, 2018, the allowance for doubtful accounts totaled $0.2 million and $0.4 million, respectively, on the condensed 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 condensed 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 September 30, 2019, and December 31, 2018, the amount included in unearned revenue, prepaid rent and other liabilities on the condensed consolidated balance sheets was approximately $1.7 million and $1.6 million, respectively. ​ Income Taxes ​ We elected to be taxed as a REIT under the Internal Revenue Code of 1986, as amended (the “Code”), commencing with our taxable year ended December 31, 2010. To qualify as a REIT, we are required to distribute at least 90% of our taxable income to our stockholders and meet various other requirements imposed by the Code relating to such matters as operating results, asset holdings, distribution levels and diversity of stock ownership. Provided we qualify for taxation as a REIT, we generally are not subject to corporate level federal income tax on the earnings distributed currently to our stockholders. If we fail to qualify as a REIT in any taxable year, and are unable to avail ourselves of certain savings provisions set forth in the Code, all of our taxable income would be subject to federal income tax at regular corporate rates. ​ To maintain REIT status, we must distribute a minimum of 90% of our taxable income. However, it is our policy and intent, subject to change, to distribute 100% of our taxable income and therefore, no provision is required in the accompanying condensed consolidated financial statements for federal income taxes with regard to our activities and our subsidiary pass-through entities. The allocable share of taxable income is included in the income tax returns of its stockholders. We are subject to the statutory requirements of the locations in which we conduct business. State and local income taxes are accrued as deemed required in the best judgment of management based on analysis and interpretation of respective tax laws. ​ We have elected to treat certain subsidiaries as taxable REIT subsidiaries (“TRS”). Certain activities that we undertake must be conducted by a TRS, such as services for our tenants that could be considered otherwise impermissible for us to perform and holding assets that we cannot hold directly. A TRS is subject to corporate level federal and state income taxes. ​ Deferred income taxes are recognized in certain taxable entities. Deferred income tax generally is a function of the period’s temporary differences (items that are treated differently for tax purposes than for financial reporting purposes), the utilization of tax net operating losses generated in prior years that previously had been recognized as deferred income tax assets and the reversal of any previously recorded deferred income tax liabilities. A valuation allowance for deferred income tax assets is provided if we believe all or some portion of the deferred income tax asset may more likely than not be realized. Any increase or decrease in the valuation allowance resulting from a change in circumstances that causes a change in the estimated realizability of the related deferred income tax asset is included in deferred tax expense. As of September 30, 2019, and December 31, 2018, the gross deferred income taxes were not material. ​ We currently have no liabilities for uncertain income tax positions. The earliest tax year for which we are subject to examination is 2016. ​ 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.

Investment in Real Estate

Investment in Real Estate	9 Months Ended
	Sep. 30, 2019
Investment in Real Estate
Investment in Real Estate	3. Investment in Real Estate ​ The following is a summary of the properties owned or leased by market at September 30, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Buildings and ​ Construction in ​ ​ ​ ​ Market Land Improvements Progress Total Cost Boston ​ $ 5,154 ​ $ 111,239 ​ $ 5,757 ​ $ 122,150 ​ Chicago ​ ​ 5,493 ​ ​ 115,265 ​ ​ 73,488 ​ ​ 194,246 ​ Denver ​ ​ — ​ ​ 32,075 ​ ​ 1,477 ​ ​ 33,552 ​ Los Angeles ​ ​ 18,672 ​ ​ 368,428 ​ ​ 52,062 ​ ​ 439,162 ​ Miami ​ ​ 728 ​ ​ 14,476 ​ ​ 124 ​ ​ 15,328 ​ New York ​ ​ 2,729 ​ ​ 155,012 ​ ​ 38,641 ​ ​ 196,382 ​ Northern Virginia ​ ​ 21,856 ​ ​ 397,131 ​ ​ 97,242 ​ ​ 516,229 ​ San Francisco Bay (1) ​ ​ 35,668 ​ ​ 703,194 ​ ​ 120,383 ​ ​ 859,245 ​ Total ​ $ 90,300 ​ $ 1,896,820 ​ $ 389,174 ​ $ 2,376,294 ​ ​ ​ (1) On April 12, 2019, we acquired a 3.8 -acre land parcel with a single-story office building located adjacent to our Santa Clara campus for a purchase price of $26 million. We expect to develop an approximately 200,000 NRSF turn-key data center building on the acquired land parcel, which we refer to as SV9, as the existing office tenants vacate upon expiration of their leases and upon the receipt of necessary entitlements. ​ The following is a summary of the properties owned or leased by market at December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Market Land Buildings and Construction in Total Cost ​ Boston ​ $ 5,154 ​ $ 107,596 ​ $ 1,644 ​ $ 114,394 ​ Chicago ​ ​ 5,493 ​ ​ 113,875 ​ ​ 14,478 ​ ​ 133,846 ​ Denver ​ ​ — ​ ​ 30,740 ​ ​ 476 ​ ​ 31,216 ​ Los Angeles ​ ​ 18,672 ​ ​ 338,011 ​ ​ 51,688 ​ ​ 408,371 ​ Miami ​ ​ 728 ​ ​ 14,014 ​ ​ 69 ​ ​ 14,811 ​ New York ​ ​ 2,729 ​ ​ 152,956 ​ ​ 33,796 ​ ​ 189,481 ​ Northern Virginia ​ ​ 22,793 ​ ​ 346,209 ​ ​ 94,623 ​ ​ 463,625 ​ San Francisco Bay ​ ​ 31,386 ​ ​ 626,928 ​ ​ 69,147 ​ ​ 727,461 ​ Total ​ $ 86,955 ​ $ 1,730,329 ​ $ 265,921 ​ $ 2,083,205 ​ ​

Other Assets

Other Assets	9 Months Ended
	Sep. 30, 2019
Other Assets.
Other Assets	4. Other Assets ​ Other assets consisted of the following, net of amortization and depreciation, if applicable for each line item, as of September 30, 2019, and December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2019 2018 Deferred rent receivable ​ $ 38,943 ​ $ 43,162 ​ Initial direct costs ​ ​ 26,891 ​ ​ 24,651 ​ Internal-use software ​ ​ 17,033 ​ ​ 17,708 ​ Prepaid expenses ​ ​ 9,117 ​ ​ 7,090 ​ Corporate furniture, fixtures and equipment ​ ​ 4,829 ​ ​ 5,006 ​ Deferred financing costs - revolving credit facility ​ ​ 2,241 ​ ​ 2,880 ​ Other ​ ​ 3,878 ​ ​ 1,793 ​ Total ​ $ 102,932 ​ $ 102,290 ​ ​

Leases

Leases	9 Months Ended
	Sep. 30, 2019
Leases
Leases	5. 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 less than 1 year to 10 years, some of the leases include options to extend the leases for up to an additional 20 years. We do not include any of our renewal options in our lease terms for calculating our lease liability as the renewal options allow us to maintain operational flexibility and we are not reasonably certain we will exercise these renewal options at this time. The weighted-average remaining non-cancelable lease term for our operating leases was nine years and ten years at September 30, 2019, and December 31, 2018, respectively. The weighted-average discount rate was 4.9% at each date. ​ During the three months ended September 30, 2019, we extended the term of approximately 5,300 NRSF at our existing DE1 space 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): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ ​ ​ 2019 ​ 2018 ​ 2019 ​ 2018 ​ Lease expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating lease expense ​ $ 6,868 ​ $ 6,144 ​ $ 19,662 ​ $ 16,789 ​ Variable lease expense ​ ​ 1,463 ​ ​ 1,185 ​ ​ 4,090 ​ ​ 3,487 ​ Rent expense ​ $ 8,331 ​ $ 7,329 ​ $ 23,752 ​ $ 20,276 ​ ​ ​

Lease Revenue

Lease Revenue	9 Months Ended
	Sep. 30, 2019
Lease Revenue
Lease Revenue	6. Lease Revenue ​ The components of data center, office, light-industrial, and other lease revenue were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ ​ ​ 2019 ​ 2018 ​ 2019 ​ 2018 ​ Lease revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum lease revenue ​ $ 103,518 ​ $ 98,294 ​ $ 304,813 ​ $ 291,488 ​ Variable lease revenue ​ ​ 22,291 ​ ​ 23,185 ​ ​ 65,605 ​ ​ 62,075 ​ Total lease revenue ​ $ 125,809 ​ $ 121,479 ​ $ 370,418 ​ $ 353,563 ​

Debt

Debt	9 Months Ended
	Sep. 30, 2019
Debt
Debt	7. Debt ​ A summary of outstanding indebtedness as of September 30, 2019, and December 31, 2018, is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maturity ​ September 30, ​ December 31, ​ ​ Interest Rate Date 2019 2018 Revolving credit facility ​ 3.47% and 3.95% at September 30, 2019, and December 31, 2018, respectively ​ April 19, 2022 ​ $ 62,250 ​ $ 211,500 ​ 2020 Senior unsecured term loan (1) ​ 3.12% and 3.37% at September 30, 2019, and December 31, 2018, respectively ​ June 24, 2020 ​ ​ 150,000 ​ ​ 150,000 ​ 2021 Senior unsecured term loan ​ 3.42% and 3.90% at September 30, 2019, and December 31, 2018, respectively ​ February 2, 2021 ​ ​ 100,000 ​ ​ 100,000 ​ 2022 Senior unsecured term loan ​ 3.42% and 3.65% at September 30, 2019, and December 31, 2018, respectively ​ April 19, 2022 ​ ​ 200,000 ​ ​ 200,000 ​ 2023 Senior unsecured term loan (2) ​ 3.77% and 4.01% at September 30, 2019, and December 31, 2018, respectively ​ April 19, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2023 Senior unsecured notes ​ 4.19% at September 30, 2019, and December 31, 2018, respectively ​ June 15, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured notes ​ 3.91% at September 30, 2019, and December 31, 2018, respectively ​ April 20, 2024 ​ ​ 175,000 ​ ​ 175,000 ​ 2026 Senior unsecured notes (3) ​ 4.52% at September 30, 2019 ​ April 17, 2026 ​ ​ 200,000 ​ ​ — ​ 2029 Senior unsecured notes ​ 4.31% at September 30, 2019 ​ April 17, 2029 ​ ​ 200,000 ​ ​ — ​ Total principal outstanding ​ ​ ​ ` ​ ​ 1,387,250 ​ ​ 1,136,500 ​ Unamortized deferred financing costs ​ ​ ​ ​ ​ ​ (6,709) ​ ​ (5,677) ​ Total debt ​ ​ ​ ​ ​ $ 1,380,541 ​ $ 1,130,823 ​ ​ ​ (1) Our Operating Partnership has in place a swap agreement with respect to the 2020 Term Loan (as defined below), effective through May 5, 2020, to swap the variable interest rate associated with $75 million, or 50% of the principal amount, of the 2020 Term Loan to a fixed rate of approximately 2.83% per annum at our current leverage ratio as of September 30, 2019. The interest rate on the remaining $75 million of the 2020 Term Loan is based on the London Interbank Offered Rate (“LIBOR”) plus the applicable spread. The effective interest rate as of September 30, 2019, is 3.12% . See Note 8 – Derivatives and Hedging Activities. (2) Our Operating Partnership has in place a swap agreement with respect to the 2023 Term Loan (as defined below), effective through April 5, 2023, to swap the variable interest rate associated with $75 million, or 50% of the principal amount of the 2023 Term Loan, to a fixed rate of approximately 4.12% per annum at our current leverage ratio as of September 30, 2019. The interest rate on the remaining $75 million of the 2023 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of September 30, 2019, is 3.77% . See Note 8 – Derivatives and Hedging Activities. (3) Our Operating Partnership had in place a $175 million forward-starting seven-year interest rate swap agreement to protect against adverse fluctuations in interest rates in anticipation of the issuance of the 2026 Notes (as defined below). On April 3, 2019, we settled the $175 million swap and after giving effect to cancellation costs incurred in connection with the termination of the swap, the 2026 Notes bear an effective interest rate of 4.52% per annum. See Note 8 – Derivatives and Hedging Activities. ​ Revolving Credit Facility ​ On April 19, 2018, our Operating Partnership and certain subsidiary co-borrowers entered into the Fourth Amended and Restated Credit Agreement (as amended and restated, the “Amended and Restated Credit Agreement”), which amended and restated our previous credit agreement to provide additional liquidity of $250 million, which was used to pay down a portion of the then-existing revolving credit facility, fund continued development across our portfolio, and for general corporate purposes. The Amended and Restated Credit Agreement, among other things, increased the revolving credit facility from $350 million to $450 million, decreased the interest rates on borrowings under the revolving credit facility and certain term loans, and extended the maturity date from June 24, 2019, to April 19, 2022, with a one-time extension option, which, if exercised, would extend the maturity date to April 19, 2023. The exercise of the extension option is subject to the payment of an extension fee equal to 10 basis points of the total commitments under the Amended and Restated Credit Agreement at initial maturity and certain other customary conditions. The Amended and Restated Credit Agreement increased our total commitment from $600 million to $850 million, consisting of a $450 million revolving credit facility, a $150 million senior unsecured term loan scheduled to mature on June 24, 2020, a $100 million senior unsecured term loan scheduled to mature on February 2, 2021, and a new $150 million senior unsecured term loan scheduled to mature on April 19, 2023. See “2020 Senior Unsecured Term Loan,” “2021 Senior Unsecured Term Loan,” and “2023 Senior Unsecured Term Loan” below for a discussion of the $150 million, $100 million, and $150 million senior unsecured term loans, respectively. The Amended and Restated Credit Agreement also increased our accordion feature by $150 million to $350 million, which allows our Operating Partnership to increase the total commitments from $850 million to $1.2 billion, under specified circumstances, including securing capital from new or existing lenders. ​ Borrowings under the revolving credit facility were amended to bear interest at a variable rate per annum equal to either (i) LIBOR plus 145 basis points to 205 basis points, or (ii) a base rate plus 45 basis points to 105 basis points, each depending on our Operating Partnership’s leverage ratio. At September 30, 2019, our Operating Partnership’s leverage ratio was 29.3% and the interest rate was LIBOR plus 145 basis points. ​ The total amount available for borrowing under the revolving credit facility, is equal to the lesser of $450.0 million or the availability calculated based on our unencumbered asset pool. As of September 30, 2019, the borrowing capacity was $450.0 million. As of September 30, 2019, $62.3 million was borrowed and outstanding, $4.9 million was outstanding under letters of credit, and therefore $382.8 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 September 30, 2019, was 29.3% ● a maximum secured debt ratio (defined as total consolidated secured debt to total gross asset value) of 40% , which, as of September 30, 2019, was 0.0% ● a minimum fixed charge coverage ratio (defined as adjusted consolidated earnings before interest, taxes, depreciation and amortization to consolidated fixed charges) of 1.5 to 1.0, which, as of September 30, 2019, was 5.9 to 1.0. ​ The Amended and Restated Credit Agreement ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, 2024 Notes, the 2026 Notes, and the 2029 Notes (each as defined herein) and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of September 30, 2019, we were in compliance with all of the financial covenants under the Amended and Restated Credit Agreement. ​ 2020 Senior Unsecured Term Loan ​ Pursuant to the terms of the Amended and Restated Credit Agreement, our Operating Partnership and certain subsidiaries are party to a $150 million senior unsecured term loan (the “2020 Term Loan”), which was originally entered into on June 24, 2015. The 2020 Term Loan has a five-year term maturing on June 24, 2020. The 2020 Term Loan ranks pari passu with the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, the 2029 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2020 Term Loan. ​ Borrowings under the 2020 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership’s leverage ratio. At September 30, 2019, the Operating Partnership’s leverage ratio was 29.3% and the interest rate was LIBOR plus 140 basis points. ​ 2021 Senior Unsecured Term Loan ​ On February 2, 2016, our Operating Partnership and certain subsidiaries partially exercised the accordion feature on our term loans under the Amended and Restated Credit Agreement and entered into a $100 million senior unsecured term loan (the “2021 Term Loan”). The 2021 Term Loan is governed by the terms of the Amended and Restated Credit Agreement. The 2021 Term Loan has a five-year term maturing on February 2, 2021. The 2021 Term Loan ranks pari passu with the 2020 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, the 2029 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2021 Term Loan. ​ Borrowings under the 2021 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership’s leverage ratio. At September 30, 2019, our Operating Partnership’s leverage ratio was 29.3% and the interest rate was LIBOR plus 140 basis points. ​ 2022 Senior Unsecured Term Loan ​ On April 19, 2017, our Operating Partnership and certain subsidiaries entered into an Amended and Restated Term Loan Agreement (as amended and restated, the “Term Loan Agreement”), which amended and restated the $100 million senior unsecured term loan, originally entered into on January 31, 2014 (the “2022 Term Loan”). The Term Loan Agreement was amended and restated to, among other things, (i) exercise the accordion feature to increase the total commitments to $200 million, (ii) extend the maturity of the term loan from January 31, 2019, to April 19, 2022, (iii) amend the accordion feature to allow an increase in total commitments from $200 million to $300 million, under specified circumstances, including securing capital from new or existing lenders, and (iv) explicitly permit the issuance of the 2024 Notes defined below (the “2024 Notes”). ​ The 2022 Term Loan ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, the 2029 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2022 Term Loan. ​ Borrowings under the 2022 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership’s leverage ratio. At September 30, 2019, our Operating Partnership’s leverage ratio was 29.3% and the interest rate was LIBOR plus 140 basis points. ​ 2023 Senior Unsecured Term Loan ​ On April 19, 2018, pursuant to the terms of, the Amended and Restated Credit Agreement, our Operating Partnership and certain subsidiaries entered into a $150 million senior unsecured term loan (the “2023 Term Loan”). The 2023 Term Loan has a five-year term maturing on April 19, 2023. The 2023 Term Loan ranks pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes, the 2029 Notes and the Amended and Restated Credit Agreement and contains the same financial covenants and other customary restrictive covenants as those debt instruments. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2023 Term Loan. ​ Borrowings under the 2023 Term Loan bear interest at a variable rate per annum equal to either (i) LIBOR plus 140 basis points to 200 basis points, or (ii) a base rate plus 40 basis points to 100 basis points, each depending on our Operating Partnership’s leverage ratio. At September 30, 2019, our Operating Partnership’s leverage ratio was 29.3% and the interest rate was LIBOR plus 140 basis points. ​ 2023 Senior Unsecured Notes ​ On June 15, 2016, our Operating Partnership issued an aggregate principal amount of $150 million, 4.19% senior unsecured notes due June 15, 2023 (the “2023 Notes”), in a private placement to certain accredited investors. The terms of the 2023 Notes are governed by a note purchase agreement, dated June 15, 2016 (the “2023 Note Purchase Agreement”), by and among our Operating Partnership, the Company and the purchasers of the 2023 Notes. ​ Interest is payable semiannually, on the 15 th ​ Our Operating Partnership may prepay all or a portion of the 2023 Notes upon notice to the holders for 100% of the principal amount plus a make-whole premium as set forth in the 2023 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2023 Notes have the right to require our Operating Partnership to purchase 100% of such holder’s 2023 Notes in cash at a purchase price equal to 100% of the principal amount thereof plus accrued and unpaid interest, if any, to the date of repurchase. ​ The 2023 Notes rank pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2024 Notes, the 2026 Notes, the 2029 Notes and the Amended and Restated Credit Agreement. On June 12, 2018, the 2023 Note Purchase Agreement was amended to, among other things, conform to the same financial covenants as the Amended and Restated Credit Agreement, as described above. In addition, certain additional financial covenants in the Amended and Restated Credit Agreement were automatically incorporated into the 2023 Note Purchase Agreement, and, subject to certain conditions, these additional financial covenants will be deleted, removed, amended or otherwise modified to be more or less restrictive if the analogous covenant in the Credit Agreement is so deleted, removed, amended or otherwise modified. These covenants are subject to a number of exceptions and qualifications set forth in the 2023 Note Purchase Agreement. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2023 Note Purchase Agreement. ​ 2024 Senior Unsecured Notes ​ On April 20, 2017, our Operating Partnership issued an aggregate principal amount of $175 million, 3.91% senior unsecured notes due April 20, 2024 (the “2024 Notes”), in a private placement to certain accredited investors. The terms of the 2024 Notes are governed by a note purchase agreement, dated April 20, 2017 (the “2024 Note Purchase Agreement”), by and among our Operating Partnership, the Company and the purchasers of the 2024 Notes. ​ Interest is payable semiannually, on the 15 th ​ Our Operating Partnership may prepay all or a portion of the 2024 Notes upon notice to the holders for 100% of the principal amount 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 would have the right to require our Operating Partnership to purchase 100% of such holders’ 2024 Notes in cash at a purchase price equal to 100% of the principal amount thereof plus accrued and unpaid interest, if any, to the date of repurchase. ​ The 2024 Notes rank pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2026 Notes, the 2029 Notes and the Amended and Restated Credit Agreement. On June 12, 2018, the 2024 Note Purchase Agreement was amended to, among other things, conform to the same financial covenants as the Amended and Restated Credit Agreement, as described above. In addition, certain additional financial covenants in the Amended and Restated Credit Agreement were automatically incorporated into the 2024 Note Purchase Agreement, and, subject to certain conditions, these additional financial covenants will be deleted, removed, amended or otherwise modified to be more or less restrictive if the analogous covenant in the Amended and Restated Credit Agreement is so deleted, removed, amended or otherwise modified. These covenants are subject to a number of exceptions and qualifications set forth in the 2024 Note Purchase Agreement. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2024 Note Purchase Agreement. ​ 2026 Senior Unsecured Notes ​ On April 17, 2019, our Operating Partnership issued an aggregate principal amount of $200 million, 4.11% Series A senior unsecured notes due April 17, 2026 (the “2026 Notes”), in a private placement to certain accredited investors. After giving effect to cancellation costs incurred in connection with the termination of an interest rate swap agreement entered into in anticipation of the issuance of the Notes, the 2026 Notes bear an effective interest rate of 4.52% per annum. The terms of the 2026 Notes are governed by a note purchase agreement, dated April 17, 2019 (the “2026 Note Purchase Agreement”), by and among our Operating Partnership, the Company and the purchasers of the 2026 Notes. ​ Interest is payable semiannually, on the 15 th ​ Our Operating Partnership may prepay all or a portion of the 2026 Notes upon notice to the holders for 100% of the principal amount plus a make-whole premium as set forth in the 2026 Note Purchase Agreement. Upon the occurrence of certain change of control events, holders of the 2026 Notes would have the right to require our Operating Partnership to purchase 100% of such holders’ 2026 Notes in cash at a purchase price equal to 100% of the principal amount thereof plus accrued and unpaid interest, if any, to the date of repurchase. ​ The 2026 Notes rank pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2024 Notes, the 2029 Notes and the Amended and Restated Credit Agreement. The 2026 Note Purchase Agreement conforms to the same financial covenants as the Amended and Restated Credit Agreement, as described above. In addition, on the date of the 2026 Note Purchase Agreement and from time to time, certain additional financial covenants in the Amended and Restated Credit Agreement will be automatically incorporated into the 2026 Note Purchase Agreement and, subject to certain conditions, will be deleted, removed, amended or otherwise modified to be more or less restrictive if the analogous covenant in the Amended and Restated Credit Agreement is so deleted, removed, amended or otherwise modified. These covenants are subject to a number of exceptions and qualifications set forth in the 2026 Note Purchase Agreement. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2026 Note Purchase Agreement. ​ 2029 Senior Unsecured Notes ​ As mentioned above, on April 17, 2019, our Operating Partnership entered into the 2026 Note Purchase Agreement to issue the 2026 Notes and an additional aggregate principal amount of $200 million, 4.31% Series B senior unsecured notes due April 17, 2029 (the “2029 Notes”), in a private placement to certain accredited investors. 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 rank pari passu with the 2020 Term Loan, the 2021 Term Loan, the 2022 Term Loan, the 2023 Term Loan, the 2023 Notes, the 2024 Notes, the 2026 Notes and the Amended and Restated Credit Agreement. As of September 30, 2019, we were in compliance with all of the financial covenants under the 2026 Note Purchase Agreement. ​ Debt Maturities ​ The following table summarizes when our debt currently becomes due (in thousands): ​ ​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2019 ​ $ — ​ 2020 ​ ​ 150,000 ​ 2021 ​ ​ 100,000 ​ 2022 ​ ​ 262,250 ​ 2023 ​ ​ 300,000 ​ Thereafter ​ ​ 575,000 ​ Total principal outstanding ​ ​ 1,387,250 ​ Unamortized deferred financing costs ​ ​ (6,709) ​ Total debt, net ​ $ 1,380,541 ​ ​

Derivatives and Hedging Activit

Derivatives and Hedging Activities	9 Months Ended
	Sep. 30, 2019
Derivatives and Hedging Activities
Derivatives and Hedging Activities	8. Derivatives and Hedging Activities ​ The following table summarizes our derivative positions as of September 30, 2019, and December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Notional Amount ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value (Level 2) (1) ​ September 30, ​ December 31, ​ Type of ​ Strike Rate ​ ​ Effective ​ Expiration ​ September 30, ​ December 31, ​ 2019 ​ 2018 ​ Derivative ​ as of 9/30/19 ​ ​ Date ​ Date ​ 2019 ​ 2018 ​ $ — ​ $ 50,000 ​ Interest Rate Swap ​ — % ​ 2/28/2014 ​ 1/31/2019 (2) $ — ​ $ 43 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 2.83 ​ ​ 5/5/2015 ​ 5/5/2020 ​ ​ 153 ​ ​ 1,099 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 4.12 ​ ​ 5/5/2018 ​ 4/5/2023 ​ ​ (3,434) ​ ​ (897) ​ ​ — ​ ​ 175,000 ​ Interest Rate Swap ​ — (3) ​ 6/3/2019 (3) 4/3/2019 (3) ​ — ​ ​ (3,183) ​ $ 150,000 ​ $ 375,000 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ (3,281) ​ $ (2,938) ​ (1) Derivative assets are recorded at fair value in our condensed consolidated balance sheets in other assets and derivative liabilities are recorded at fair value in our condensed consolidated balance sheets in unearned revenue, prepaid rent and other liabilities. We do not net our derivative position by counterparty for purposes of balance sheet presentation and disclosure. (2) On January 31, 2019, the remaining $50 million of a five-year interest rate swap agreement, which reduced our variability in cash flows relating to interest payments based on one-month LIBOR variable rate debt, expired. (3) On December 6, 2018, we entered into forward-starting seven-year interest rate swap agreement to protect against adverse fluctuations in interest rates. The swap effectively fixed the interest rate at approximately 2.91% per annum plus the applicable spread. On April 3, 2019, we settled the $175 million forward-starting seven-year interest rate swap, in connection with the anticipated subsequent debt issuance of $200 million of the 2026 Notes, at a loss of $5.7 million. The loss is included in accumulated other comprehensive income and will be amortized to interest expense over the term of the 2026 Notes. ​ 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 condensed consolidated balance sheets and is subsequently reclassified into earnings in the period that the hedged forecasted transaction affects earnings. ​ Amounts reported in accumulated other comprehensive income 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 October 1, 2019, we estimate that $1.5 million will be reclassified as an increase to interest expense.

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2019
Stockholders' Equity
Stockholders' Equity	9. Stockholders’ Equity ​ We announced the following dividends per share on our common stock during the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Declaration Date Record Date Payment Date Common Stock March 7, 2019 ​ March 29, 2019 ​ April 15, 2019 ​ $ 1.10 ​ May 16, 2019 ​ June 28, 2019 ​ July 15, 2019 ​ ​ 1.22 ​ September 4, 2019 ​ September 30, 2019 ​ October 15, 2019 ​ ​ 1.22 ​ Total ​ ​ ​ ​ ​ $ 3.54 ​ ​

Noncontrolling Interests - Oper

Noncontrolling Interests - Operating Partnership	9 Months Ended
	Sep. 30, 2019
Noncontrolling Interests - Operating Partnership
Noncontrolling Interests - Operating Partnership	10. Noncontrolling Interests — Operating Partnership ​ Noncontrolling interests represent the limited partnership interests in our Operating Partnership held by individuals and entities other than CoreSite Realty Corporation. The current holders of common Operating Partnership units are eligible to have the common Operating Partnership units redeemed for cash or common stock on a one-for-one basis, at our option. ​ The following table shows the common ownership interests in our Operating Partnership as of September 30, 2019, and December 31, 2018: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 ​ December 31, 2018 ​ ​ Number of Units Percentage of Total ​ Number of Units Percentage of Total ​ CoreSite Realty Corporation ​ 37,230,006 ​ 77.6 % 36,256,032 ​ 75.8 % Noncontrolling interests ​ 10,771,658 ​ 22.4 ​ 11,575,268 ​ 24.2 ​ Total ​ 48,001,664 ​ 100.0 % 47,831,300 ​ 100.0 % ​ For each share of common stock issued by us, our Operating Partnership issues to us an equivalent common Operating Partnership unit. During the nine months ended September 30, 2019, we issued 170,364 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 received aggregate distributions of $3.54 per unit during the nine months ended September 30, 2019, payable in correlation with declared dividends on shares of our common stock. ​ During the nine months ended September 30, 2019, 803,610 common Operating Partnership units held by affiliates of The Carlyle Group and other third parties were redeemed for shares of our common stock in connection with the sale of 803,610 shares of our common stock. This redemption was recorded as a $5.2 million reduction to noncontrolling interests in our Operating Partnership and an increase to total stockholder’s equity in the condensed consolidated balance sheets. ​ The redemption value of the noncontrolling interests at September 30, 2019, was $1.3 billion based on the closing price of the Company’s common stock of $121.85 per share on the last trading day prior to that date.

Equity Incentive Plan

Equity Incentive Plan	9 Months Ended
	Sep. 30, 2019
Equity Incentive Award Plan
Equity Incentive Plan	11. Equity Incentive Plan ​ Our Board of Directors adopted and, with the approval of our stockholders, amended the 2010 Equity Incentive Plan (as amended, the “2010 Plan”) in 2013. The 2010 Plan is administered by the Compensation Committee of our Board of Directors. Awards issuable under the 2010 Plan include common stock, stock options, restricted stock, stock appreciation rights, dividend equivalents, Operating Partnership units and other incentive awards. We have reserved a total of 6,000,000 shares of our common stock for issuance pursuant to the 2010 Plan, which may be adjusted for changes in our capitalization and certain corporate transactions. To the extent that an award expires, terminates or lapses, or an award is settled in cash without the delivery of shares of common stock to the participant, then any unvested shares subject to the award will be available for future grant or sale under the 2010 Plan. Shares of restricted stock that are forfeited or repurchased by us pursuant to the 2010 Plan may again be awarded under the 2010 Plan. The payment of dividend equivalents in cash in conjunction with any outstanding awards will not be counted against the shares available for issuance under the 2010 Plan. ​ As of September 30, 2019, 2,674,453 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. As of September 30, 2019, all stock option awards are fully vested. The following table sets forth stock option activity under the 2010 Plan for the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ ​ ​ ​ ​ Shares ​ Weighted- ​ ​ ​ Subject to ​ Average ​ ​ ​ Option ​ Exercise Price ​ Options outstanding, December 31, 2018 49,133 $ 20.06 Granted ​ — ​ ​ — ​ Exercised ​ (3,622) ​ ​ 18.10 ​ Forfeited ​ — ​ ​ — ​ Expired ​ — ​ ​ — ​ Options outstanding, September 30, 2019 ​ 45,511 ​ $ 20.21 ​ ​ Restricted Stock Awards and Units ​ Restricted stock awards and restricted stock units, or RSUs, are granted with a fair value equal to the closing market price of the Company’s common stock on the date of grant. The principal difference between restricted stock awards and RSUs is that RSUs are not shares of our common stock and do not have any of the rights or privileges thereof, including voting rights. On the applicable vesting date, the holder of an RSU becomes entitled to a share of common stock. The restricted stock awards and RSUs are amortized on a straight-line basis to expense over the vesting period. The following table sets forth the number of unvested restricted stock awards and RSUs and the weighted-average fair value of these awards at the date of grant: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Restricted ​ Weighted- ​ ​ ​ Stock ​ Average Fair ​ ​ ​ Awards and ​ Value at Grant ​ ​ Units Date ​ Unvested balance, December 31, 2018 ​ 284,879 ​ $ 85.98 ​ Granted ​ 158,608 ​ ​ 100.32 ​ Forfeited ​ (19,865) ​ ​ 93.65 ​ Vested ​ (129,008) ​ ​ 78.81 ​ Unvested balance, September 30, 2019 ​ 294,614 ​ $ 96.32 ​ ​ As of September 30, 2019, total unearned compensation on restricted stock awards and RSUs was approximately $22.1 million, and the weighted-average vesting period was 2.5 years. ​ Performance Stock Awards ​ We grant long-term incentives to members of management in the form of performance-based restricted stock awards (“PSAs”) under the 2010 Plan. The number of PSAs earned is based on our achievement of relative total shareholder return (“TSR”) measured versus the MSCI US REIT Index over a three-year performance period and ranges between 25% and 175% of the target number of shares for PSAs granted in 2017, 2018, and 2019. The PSAs are granted at the maximum percentage of target and are retired annually to the extent we do not meet the maximum relative TSR performance threshold versus the MSCI US REIT Index. The PSAs are earned upon TSR achievement measured both annually and over the full three-year performance period. The PSAs have a service condition and will be released at the end of the three-year performance period, to the extent earned, provided that the holder continues to be employed or otherwise in service of the Company at the end of the three-year performance period. The PSAs are amortized on a straight-line basis to expense over the vesting period. Holders of the PSAs are entitled to dividends on the PSAs, which are accrued and paid in cash at the end of the three-year performance period. ​ The following table sets forth the number of unvested PSAs and the weighted-average fair value of these awards at the date of grant: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ ​ ​ ​ Average Fair ​ ​ ​ Performance-Based Restricted Stock Awards ​ Value at Grant ​ ​ Minimum ​ Maximum Target Date Unvested balance, December 31, 2018 ​ 50,823 ​ 173,944 ​ 112,385 ​ $ 96.58 ​ Granted ​ 10,926 ​ 76,480 ​ 43,703 ​ ​ 114.55 ​ Performance adjustment (1) ​ 18,063 ​ (29,819) ​ (5,878) ​ ​ — ​ Forfeited ​ (1,538) ​ (5,604) ​ (3,571) ​ ​ 103.26 ​ Vested ​ (43,650) ​ (43,650) ​ (43,650) ​ ​ 84.78 ​ Unvested balance, September 30, 2019 ​ 34,624 ​ 171,351 ​ 102,989 ​ $ 107.84 ​ (1) Includes the annual adjustment for the number of PSAs earned based on our achievement of relative TSR measured versus the MSCI US REIT Index for the applicable performance periods. ​ As of September 30, 2019, total unearned compensation on PSAs was approximately $6.5 million, and the weighted-average vesting period was 2. ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2019 ​ 2018 ​ Expected term (in years) ​ 2.82 ​ 2.82 ​ Expected volatility ​ 24.09 % 24.15 % Expected annual dividend (1) ​ — ​ — ​ Risk-free rate ​ 2.48 % 2.24 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share

Earnings Per Share	9 Months Ended
	Sep. 30, 2019
Earnings Per Share
Earnings Per Share	​ 12. 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): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ ​ 2019 2018 2019 2018 Net income attributable to common shares ​ $ 17,450 ​ $ 18,600 ​ $ 56,646 ​ $ 58,291 ​ Weighted-average common shares outstanding - basic ​ ​ 36,950,950 ​ ​ 35,512,091 ​ ​ 36,589,593 ​ ​ 34,504,790 ​ Effect of potentially dilutive common shares: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options ​ ​ 38,172 ​ ​ 45,453 ​ ​ 38,663 ​ ​ 49,349 ​ Unvested awards ​ ​ 143,033 ​ ​ 163,934 ​ ​ 134,985 ​ ​ 139,696 ​ Weighted-average common shares outstanding - diluted ​ ​ 37,132,155 ​ ​ 35,721,478 ​ ​ 36,763,241 ​ ​ 34,693,835 ​ Net income per share attributable to common shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ $ 0.47 ​ $ 0.52 ​ $ 1.55 ​ $ 1.69 ​ Diluted ​ $ 0.47 ​ $ 0.52 ​ $ 1.54 ​ $ 1.68 ​ ​ In the calculations above, we have excluded weighted-average potentially dilutive securities of 100 and 1,003 for the three months ended September 30, 2019, and 2018, respectively, 1,810 and 121,373 for the nine months ended September 30, 2019, and 2018, respectively, as their effect would have been antidilutive.

Estimated Fair Value of Financi

Estimated Fair Value of Financial Instruments	9 Months Ended
	Sep. 30, 2019
Estimated Fair Value of Financial Instruments
Estimated Fair Value of Financial Instruments	13. 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 September 30, 2019, we have assessed the significance of the impact of the credit valuation adjustments on the overall valuation of our derivative positions and have determined that the credit valuation adjustment is not significant to the overall valuation of our derivative portfolio. As a result, we classify our derivative valuation in Level 2 of the fair value hierarchy. ​ The total principal balance of our revolving credit facility, senior unsecured term loans, and senior unsecured notes was $1.4 billion and $1.1 billion as of September 30, 2019, and December 31, 2018, 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	9 Months Ended
	Sep. 30, 2019
Commitments and Contingencies.
Commitments and Contingencies	14. 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 September 30, 2019, we had open commitments related to construction contracts of approximately $216.3 million. ​ Additionally, we have commitments related to telecommunications capacity used to connect data centers located within the same market or geographical area, power usage, and company-wide improvements that are ancillary to revenue generation. At September 30, 2019, we had open commitments related to these contracts of approximately $73.8 million, of which $3.3 million is scheduled to be met during the remainder of the year ending December 31, 2019. ​ On April 27, 2018, we filed suit against DGEB Management, LLC and its affiliates, the landlord of our DE1 facility (the “Landlord”), in the District Court, City and County of Denver, State of Colorado (the “District Court”), for certain claims relating to the building where our DE1 facility is located. The parties asserted various claims and counterclaims related to three primary areas: (1) a construction project within the DE1 building to house electrical transformers used to supply power, (2) our usage of images of the DE1 building on our website, and (3) the protocol for cross-connections within the DE1 building. The Landlord demanded approximately $21 million in damages, which included approximately $2.8 million related to the construction project, and further sought to hold us in default under the lease. On August 22, 2019, the jury found in our favor on all claims and counterclaims on which it was asked to decide, and awarded us nominal damages. Upon unopposed application, the District Court vacated deadlines for post-trial matters, including our application for reimbursement of legal fees under the lease, to provide time for the parties to discuss settlement of these post-trial matters. If the parties are unable to reach a settlement, we will request that the District Court reinstate these post-trial deadlines. We estimate a minimum reimbursement in attorneys’ fees and costs of $3.1 million, which was recorded in other assets on our condensed consolidated balance sheet. ​ In the ordinary course of business, we are subject to claims and administrative proceedings. Except as described above, we are not presently party to any proceeding, which we believe to be material or which we would expect to have, individually or in the aggregate, a material adverse effect on our business, financial condition, cash flows or results of operations. The outcome of litigation and administrative proceedings is inherently uncertain. Therefore, if one or more legal or administrative matters are resolved against us in a reporting period for amounts in excess of management’s expectations, our financial condition, cash flows or results of operations for that reporting period could be materially adversely affected.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2019
Summary of Significant Accounting Policies
Principles of Consolidation and Basis of Presentation	Principles of Consolidation and Basis of Presentation ​ The accompanying unaudited condensed consolidated financial statements have been prepared by our management in accordance with U.S. generally accepted accounting principles (“GAAP”) for interim financial information and in compliance with the rules and regulations of the U.S. Securities and Exchange Commission. Accordingly, these unaudited condensed consolidated financial statements do not include all of the information and footnotes required by GAAP for complete financial statements. In the opinion of our management, all adjustments (consisting of normal recurring adjustments) considered necessary for a fair presentation have been included. The results of operations for the three and nine months ended September 30, 2019, are not necessarily indicative of the expected results for the year ending December 31, 2019, or any other future period. These unaudited condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and the notes thereto included in our Annual Report on Form 10-K ​ Our Operating Partnership meets the definition and criteria of a variable interest entity (“VIE”) and we are the primary beneficiary of the VIE. Our sole significant asset is the investment in our Operating Partnership, and consequently, substantially all of our assets and liabilities represent those assets and liabilities of our Operating Partnership. Our debt is an obligation of our Operating Partnership where the creditors also have recourse against the credit of the Company. Intercompany balances and transactions have been eliminated upon consolidation.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Not Yet Adopted ​ 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 We do not expect the provisions of ASU 2018-15 to have a material impact on our condensed consolidated financial statements. We determined that all other recently issued accounting pronouncements will not have a material impact on our consolidated financial statements or do not apply to our operations.
Use of Estimates	Use of Estimates ​ The preparation of these unaudited condensed 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 unaudited condensed 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 $3.7 million and $1.3 million for the three months ended September 30, 2019, and 2018, respectively. Capitalized interest costs were $10.0 million and $3.5 million for the nine months ended September 30, 2019, and 2018, respectively. ​ Depreciation and amortization are calculated using the straight-line method over the following useful lives of the assets: ​ ​ ​ ​ Buildings 27 to 40 years Building improvements ​ 1 to 10 years Leasehold improvements ​ The shorter of the lease term or useful life of the asset ​ Depreciation expense was $35.1 million and $31.4 million for the three months ended September 30, 2019, and 2018, respectively. Depreciation expense was $100.1 million and $91.3 million for the nine months ended September 30, 2019, and 2018, respectively.
Acquisition of Investment in Real Estate	Acquisition of Investment in Real Estate ​ When accounting for business combinations and asset acquisitions, the fair value of the real estate acquired is allocated to the acquired tangible assets, consisting primarily of land, building and building improvements, and identified intangible assets and liabilities, consisting of the value of above-market and below-market leases, value of in-place leases and the value of customer relationships. The primary difference between business combinations and asset acquisitions is that asset acquisitions require cost accumulation and allocation at a relative fair value. Acquisition costs are capitalized for asset acquisitions and 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 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 three or nine months ended September 30, 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 September 30, 2019, and December 31, 2018, we had $40.6 million of goodwill at each date. The Company’s goodwill has an indeterminate life and is not amortized, but is tested for impairment on an annual basis, or more frequently if events or changes in circumstances indicate that the asset might be impaired. No impairment loss was recognized for the three or nine months ended September 30, 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 $4.0 million for both the three months ended September 30, 2019, and 2018. Amortization of initial direct costs was $10.8 million and $11.8 million for the nine months ended September 30, 2019, and 2018, respectively. Initial direct costs are included within other assets in the condensed consolidated balance sheets and consisted of the following, net of amortization, as of September 30, 2019, and December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2019 2018 Internal sales commissions ​ $ 15,340 ​ $ 14,199 ​ Third party commissions ​ ​ 11,066 ​ ​ 9,855 ​ Other ​ ​ 485 ​ ​ 597 ​ Total ​ $ 26,891 ​ $ 24,651 ​
Deferred Financing Costs	Deferred Financing Costs ​ Deferred financing costs include costs incurred in connection with obtaining debt and extending existing debt. These financing costs are capitalized and amortized on a straight-line basis, which approximates the effective-interest method, over the term of the indebtedness and the amortization is included as a component of interest expense. Depending on the type of debt instrument, deferred financing costs are reported either in other assets or as a direct deduction from the carrying amount of the related debt liabilities in our condensed consolidated balance sheets.
Recoverability of Long-Lived Assets	Recoverability of Long-Lived Assets ​ We review our long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Impairment is recognized when estimated expected future cash flows (undiscounted and without interest charges) are less than the carrying amount of the assets. The estimation of expected future net cash flows is inherently uncertain and relies, to a considerable extent, on assumptions regarding current and future economics and market conditions and the availability of capital. If, in future periods, there are changes in the estimates or assumptions incorporated into the impairment review analysis, the changes could result in an adjustment to the carrying amount of the long-lived assets. To the extent that impairment has occurred, the excess of the carrying amount of long-lived assets over its estimated fair value would be recognized as an impairment loss charged to net income. For the three and nine months ended September 30, 2019, and 2018, no impairment of long-lived assets was recognized in the condensed 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 condensed 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 8, 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. No impairment of internal-use software was recognized in the condensed consolidated financial statements for the three and nine months ended September 30, 2019, and 2018.
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 condensed 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 condensed consolidated statements of operations. For fixed power arrangements, a customer pays us a fixed monthly fee for a committed available amount of power. We recognize the fixed power revenue each month over the term of the lease. For variable power arrangements, a customer pays us variable monthly fees for the specific amount of power utilized at the current utility rates. We recognize variable power revenue each month as the uncertainty related to the consideration is resolved, as power is provided to our customers, and as our customers utilize the power. ​ Some of our leases contain provisions under which our customers reimburse us for common area maintenance and other executory costs. These customer reimbursements are variable and are recognized in the period that the expenses are recognized. These services have the same pattern of transfer over the lease term and are also combined with lease revenue within our condensed consolidated statements of operations. ​ Interconnection Revenue ​ We also derive revenue from interconnection services, which are generally contracted on a month-to-month basis cancellable by us or the customer at any time. Interconnection services are accounted for as separate contracts and are not combined with lease and power arrangements. We recognize interconnection revenue each month as these services are delivered to, and utilized by, our customers. ​ Allowance for Doubtful Accounts ​ A provision for uncollectible accounts is recorded if a receivable balance relating to contractual rent, rental revenue recorded on a straight-line basis, tenant reimbursements or other billed amounts is considered by management to be uncollectible. At September 30, 2019, and December 31, 2018, the allowance for doubtful accounts totaled $0.2 million and $0.4 million, respectively, on the condensed 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 condensed 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 September 30, 2019, and December 31, 2018, the amount included in unearned revenue, prepaid rent and other liabilities on the condensed consolidated balance sheets was approximately $1.7 million and $1.6 million, respectively.
Income Taxes	Income Taxes ​ We elected to be taxed as a REIT under the Internal Revenue Code of 1986, as amended (the “Code”), commencing with our taxable year ended December 31, 2010. To qualify as a REIT, we are required to distribute at least 90% of our taxable income to our stockholders and meet various other requirements imposed by the Code relating to such matters as operating results, asset holdings, distribution levels and diversity of stock ownership. Provided we qualify for taxation as a REIT, we generally are not subject to corporate level federal income tax on the earnings distributed currently to our stockholders. If we fail to qualify as a REIT in any taxable year, and are unable to avail ourselves of certain savings provisions set forth in the Code, all of our taxable income would be subject to federal income tax at regular corporate rates. ​ To maintain REIT status, we must distribute a minimum of 90% of our taxable income. However, it is our policy and intent, subject to change, to distribute 100% of our taxable income and therefore, no provision is required in the accompanying condensed consolidated financial statements for federal income taxes with regard to our activities and our subsidiary pass-through entities. The allocable share of taxable income is included in the income tax returns of its stockholders. We are subject to the statutory requirements of the locations in which we conduct business. State and local income taxes are accrued as deemed required in the best judgment of management based on analysis and interpretation of respective tax laws. ​ We have elected to treat certain subsidiaries as taxable REIT subsidiaries (“TRS”). Certain activities that we undertake must be conducted by a TRS, such as services for our tenants that could be considered otherwise impermissible for us to perform and holding assets that we cannot hold directly. A TRS is subject to corporate level federal and state income taxes. ​ Deferred income taxes are recognized in certain taxable entities. Deferred income tax generally is a function of the period’s temporary differences (items that are treated differently for tax purposes than for financial reporting purposes), the utilization of tax net operating losses generated in prior years that previously had been recognized as deferred income tax assets and the reversal of any previously recorded deferred income tax liabilities. A valuation allowance for deferred income tax assets is provided if we believe all or some portion of the deferred income tax asset may more likely than not be realized. Any increase or decrease in the valuation allowance resulting from a change in circumstances that causes a change in the estimated realizability of the related deferred income tax asset is included in deferred tax expense. As of September 30, 2019, and December 31, 2018, the gross deferred income taxes were not material. ​ We currently have no liabilities for uncertain income tax positions. The earliest tax year for which we are subject to examination is 2016.
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)	9 Months Ended
	Sep. 30, 2019
Summary of Significant Accounting Policies
Schedule of useful lives of real estate investment assets	​ ​ ​ ​ Buildings 27 to 40 years Building improvements ​ 1 to 10 years Leasehold improvements ​ The shorter of the lease term or useful life of the asset
Schedule of deferred leasing costs, net of amortization	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2019 2018 Internal sales commissions ​ $ 15,340 ​ $ 14,199 ​ Third party commissions ​ ​ 11,066 ​ ​ 9,855 ​ Other ​ ​ 485 ​ ​ 597 ​ Total ​ $ 26,891 ​ $ 24,651 ​

Investment in Real Estate (Tabl

Investment in Real Estate (Tables)	9 Months Ended
	Sep. 30, 2019
Investment in Real Estate
Summary of properties owned or leased	The following is a summary of the properties owned or leased by market at September 30, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Buildings and ​ Construction in ​ ​ ​ ​ Market Land Improvements Progress Total Cost Boston ​ $ 5,154 ​ $ 111,239 ​ $ 5,757 ​ $ 122,150 ​ Chicago ​ ​ 5,493 ​ ​ 115,265 ​ ​ 73,488 ​ ​ 194,246 ​ Denver ​ ​ — ​ ​ 32,075 ​ ​ 1,477 ​ ​ 33,552 ​ Los Angeles ​ ​ 18,672 ​ ​ 368,428 ​ ​ 52,062 ​ ​ 439,162 ​ Miami ​ ​ 728 ​ ​ 14,476 ​ ​ 124 ​ ​ 15,328 ​ New York ​ ​ 2,729 ​ ​ 155,012 ​ ​ 38,641 ​ ​ 196,382 ​ Northern Virginia ​ ​ 21,856 ​ ​ 397,131 ​ ​ 97,242 ​ ​ 516,229 ​ San Francisco Bay (1) ​ ​ 35,668 ​ ​ 703,194 ​ ​ 120,383 ​ ​ 859,245 ​ Total ​ $ 90,300 ​ $ 1,896,820 ​ $ 389,174 ​ $ 2,376,294 ​ ​ ​ (1) On April 12, 2019, we acquired a 3.8 -acre land parcel with a single-story office building located adjacent to our Santa Clara campus for a purchase price of $26 million. We expect to develop an approximately 200,000 NRSF turn-key data center building on the acquired land parcel, which we refer to as SV9, as the existing office tenants vacate upon expiration of their leases and upon the receipt of necessary entitlements. ​ The following is a summary of the properties owned or leased by market at December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Market Land Buildings and Construction in Total Cost ​ Boston ​ $ 5,154 ​ $ 107,596 ​ $ 1,644 ​ $ 114,394 ​ Chicago ​ ​ 5,493 ​ ​ 113,875 ​ ​ 14,478 ​ ​ 133,846 ​ Denver ​ ​ — ​ ​ 30,740 ​ ​ 476 ​ ​ 31,216 ​ Los Angeles ​ ​ 18,672 ​ ​ 338,011 ​ ​ 51,688 ​ ​ 408,371 ​ Miami ​ ​ 728 ​ ​ 14,014 ​ ​ 69 ​ ​ 14,811 ​ New York ​ ​ 2,729 ​ ​ 152,956 ​ ​ 33,796 ​ ​ 189,481 ​ Northern Virginia ​ ​ 22,793 ​ ​ 346,209 ​ ​ 94,623 ​ ​ 463,625 ​ San Francisco Bay ​ ​ 31,386 ​ ​ 626,928 ​ ​ 69,147 ​ ​ 727,461 ​ Total ​ $ 86,955 ​ $ 1,730,329 ​ $ 265,921 ​ $ 2,083,205 ​ ​

Other Assets (Tables)

Other Assets (Tables)	9 Months Ended
	Sep. 30, 2019
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 September 30, 2019, and December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2019 2018 Deferred rent receivable ​ $ 38,943 ​ $ 43,162 ​ Initial direct costs ​ ​ 26,891 ​ ​ 24,651 ​ Internal-use software ​ ​ 17,033 ​ ​ 17,708 ​ Prepaid expenses ​ ​ 9,117 ​ ​ 7,090 ​ Corporate furniture, fixtures and equipment ​ ​ 4,829 ​ ​ 5,006 ​ Deferred financing costs - revolving credit facility ​ ​ 2,241 ​ ​ 2,880 ​ Other ​ ​ 3,878 ​ ​ 1,793 ​ Total ​ $ 102,932 ​ $ 102,290 ​

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2019
Leases
Schedule of components of lease expense	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ ​ ​ 2019 ​ 2018 ​ 2019 ​ 2018 ​ Lease expense: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Operating lease expense ​ $ 6,868 ​ $ 6,144 ​ $ 19,662 ​ $ 16,789 ​ Variable lease expense ​ ​ 1,463 ​ ​ 1,185 ​ ​ 4,090 ​ ​ 3,487 ​ Rent expense ​ $ 8,331 ​ $ 7,329 ​ $ 23,752 ​ $ 20,276 ​

Lease Revenue (Tables)

Lease Revenue (Tables)	9 Months Ended
	Sep. 30, 2019
Lease Revenue
Components of data center, office, light-industrial, and other lease revenue	The components of data center, office, light-industrial, and other lease revenue were as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ ​ ​ 2019 ​ 2018 ​ 2019 ​ 2018 ​ Lease revenue: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Minimum lease revenue ​ $ 103,518 ​ $ 98,294 ​ $ 304,813 ​ $ 291,488 ​ Variable lease revenue ​ ​ 22,291 ​ ​ 23,185 ​ ​ 65,605 ​ ​ 62,075 ​ Total lease revenue ​ $ 125,809 ​ $ 121,479 ​ $ 370,418 ​ $ 353,563 ​

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2019
Debt
Summary of outstanding indebtedness	​ A summary of outstanding indebtedness as of September 30, 2019, and December 31, 2018, is as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Maturity ​ September 30, ​ December 31, ​ ​ Interest Rate Date 2019 2018 Revolving credit facility ​ 3.47% and 3.95% at September 30, 2019, and December 31, 2018, respectively ​ April 19, 2022 ​ $ 62,250 ​ $ 211,500 ​ 2020 Senior unsecured term loan (1) ​ 3.12% and 3.37% at September 30, 2019, and December 31, 2018, respectively ​ June 24, 2020 ​ ​ 150,000 ​ ​ 150,000 ​ 2021 Senior unsecured term loan ​ 3.42% and 3.90% at September 30, 2019, and December 31, 2018, respectively ​ February 2, 2021 ​ ​ 100,000 ​ ​ 100,000 ​ 2022 Senior unsecured term loan ​ 3.42% and 3.65% at September 30, 2019, and December 31, 2018, respectively ​ April 19, 2022 ​ ​ 200,000 ​ ​ 200,000 ​ 2023 Senior unsecured term loan (2) ​ 3.77% and 4.01% at September 30, 2019, and December 31, 2018, respectively ​ April 19, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2023 Senior unsecured notes ​ 4.19% at September 30, 2019, and December 31, 2018, respectively ​ June 15, 2023 ​ ​ 150,000 ​ ​ 150,000 ​ 2024 Senior unsecured notes ​ 3.91% at September 30, 2019, and December 31, 2018, respectively ​ April 20, 2024 ​ ​ 175,000 ​ ​ 175,000 ​ 2026 Senior unsecured notes (3) ​ 4.52% at September 30, 2019 ​ April 17, 2026 ​ ​ 200,000 ​ ​ — ​ 2029 Senior unsecured notes ​ 4.31% at September 30, 2019 ​ April 17, 2029 ​ ​ 200,000 ​ ​ — ​ Total principal outstanding ​ ​ ​ ` ​ ​ 1,387,250 ​ ​ 1,136,500 ​ Unamortized deferred financing costs ​ ​ ​ ​ ​ ​ (6,709) ​ ​ (5,677) ​ Total debt ​ ​ ​ ​ ​ $ 1,380,541 ​ $ 1,130,823 ​ ​ ​ (1) Our Operating Partnership has in place a swap agreement with respect to the 2020 Term Loan (as defined below), effective through May 5, 2020, to swap the variable interest rate associated with $75 million, or 50% of the principal amount, of the 2020 Term Loan to a fixed rate of approximately 2.83% per annum at our current leverage ratio as of September 30, 2019. The interest rate on the remaining $75 million of the 2020 Term Loan is based on the London Interbank Offered Rate (“LIBOR”) plus the applicable spread. The effective interest rate as of September 30, 2019, is 3.12% . See Note 8 – Derivatives and Hedging Activities. (2) Our Operating Partnership has in place a swap agreement with respect to the 2023 Term Loan (as defined below), effective through April 5, 2023, to swap the variable interest rate associated with $75 million, or 50% of the principal amount of the 2023 Term Loan, to a fixed rate of approximately 4.12% per annum at our current leverage ratio as of September 30, 2019. The interest rate on the remaining $75 million of the 2023 Term Loan is based on LIBOR plus the applicable spread. The effective interest rate as of September 30, 2019, is 3.77% . See Note 8 – Derivatives and Hedging Activities. (3) Our Operating Partnership had in place a $175 million forward-starting seven-year interest rate swap agreement to protect against adverse fluctuations in interest rates in anticipation of the issuance of the 2026 Notes (as defined below). On April 3, 2019, we settled the $175 million swap and after giving effect to cancellation costs incurred in connection with the termination of the swap, the 2026 Notes bear an effective interest rate of 4.52% per annum. See Note 8 – Derivatives and Hedging Activities.
Summary of amount of the entity's outstanding debt when such debt currently become due	​ ​ ​ ​ ​ ​ Year Ending December 31, ​ ​ 2019 ​ $ — ​ 2020 ​ ​ 150,000 ​ 2021 ​ ​ 100,000 ​ 2022 ​ ​ 262,250 ​ 2023 ​ ​ 300,000 ​ Thereafter ​ ​ 575,000 ​ Total principal outstanding ​ ​ 1,387,250 ​ Unamortized deferred financing costs ​ ​ (6,709) ​ Total debt, net ​ $ 1,380,541 ​

Derivatives and Hedging Activ_2

Derivatives and Hedging Activities (Tables)	9 Months Ended
	Sep. 30, 2019
Derivatives and Hedging Activities
Schedule of derivative instruments	The following table summarizes our derivative positions as of September 30, 2019, and December 31, 2018 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Notional Amount ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value (Level 2) (1) ​ September 30, ​ December 31, ​ Type of ​ Strike Rate ​ ​ Effective ​ Expiration ​ September 30, ​ December 31, ​ 2019 ​ 2018 ​ Derivative ​ as of 9/30/19 ​ ​ Date ​ Date ​ 2019 ​ 2018 ​ $ — ​ $ 50,000 ​ Interest Rate Swap ​ — % ​ 2/28/2014 ​ 1/31/2019 (2) $ — ​ $ 43 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 2.83 ​ ​ 5/5/2015 ​ 5/5/2020 ​ ​ 153 ​ ​ 1,099 ​ ​ 75,000 ​ ​ 75,000 ​ Interest Rate Swap ​ 4.12 ​ ​ 5/5/2018 ​ 4/5/2023 ​ ​ (3,434) ​ ​ (897) ​ ​ — ​ ​ 175,000 ​ Interest Rate Swap ​ — (3) ​ 6/3/2019 (3) 4/3/2019 (3) ​ — ​ ​ (3,183) ​ $ 150,000 ​ $ 375,000 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ (3,281) ​ $ (2,938) ​ (1) Derivative assets are recorded at fair value in our condensed consolidated balance sheets in other assets and derivative liabilities are recorded at fair value in our condensed consolidated balance sheets in unearned revenue, prepaid rent and other liabilities. We do not net our derivative position by counterparty for purposes of balance sheet presentation and disclosure. (2) On January 31, 2019, the remaining $50 million of a five-year interest rate swap agreement, which reduced our variability in cash flows relating to interest payments based on one-month LIBOR variable rate debt, expired. (3) On December 6, 2018, we entered into forward-starting seven-year interest rate swap agreement to protect against adverse fluctuations in interest rates. The swap effectively fixed the interest rate at approximately 2.91% per annum plus the applicable spread. On April 3, 2019, we settled the $175 million forward-starting seven-year interest rate swap, in connection with the anticipated subsequent debt issuance of $200 million of the 2026 Notes, at a loss of $5.7 million. The loss is included in accumulated other comprehensive income and will be amortized to interest expense over the term of the 2026 Notes.

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2019
Stockholders' Equity
Schedule of dividends paid per share on Series A Cumulative Preferred Stock and common shares	We announced the following dividends per share on our common stock during the nine months ended September 30, 2019: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Declaration Date Record Date Payment Date Common Stock March 7, 2019 ​ March 29, 2019 ​ April 15, 2019 ​ $ 1.10 ​ May 16, 2019 ​ June 28, 2019 ​ July 15, 2019 ​ ​ 1.22 ​ September 4, 2019 ​ September 30, 2019 ​ October 15, 2019 ​ ​ 1.22 ​ Total ​ ​ ​ ​ ​ $ 3.54 ​

Noncontrolling Interests - Op_2

Noncontrolling Interests - Operating Partnership (Tables)	9 Months Ended
	Sep. 30, 2019
Noncontrolling Interests - Operating Partnership
Schedule of ownership interests in our Operating Partnership	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2019 ​ December 31, 2018 ​ ​ Number of Units Percentage of Total ​ Number of Units Percentage of Total ​ CoreSite Realty Corporation ​ 37,230,006 ​ 77.6 % 36,256,032 ​ 75.8 % Noncontrolling interests ​ 10,771,658 ​ 22.4 ​ 11,575,268 ​ 24.2 ​ Total ​ 48,001,664 ​ 100.0 % 47,831,300 ​ 100.0 %

Equity Incentive Award Plan (Ta

Equity Incentive Award Plan (Tables)	9 Months Ended
	Sep. 30, 2019
Equity Incentive Award Plan
Schedule of stock option activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ ​ ​ ​ ​ Shares ​ Weighted- ​ ​ ​ Subject to ​ Average ​ ​ ​ Option ​ Exercise Price ​ Options outstanding, December 31, 2018 49,133 $ 20.06 Granted ​ — ​ ​ — ​ Exercised ​ (3,622) ​ ​ 18.10 ​ Forfeited ​ — ​ ​ — ​ Expired ​ — ​ ​ — ​ Options outstanding, September 30, 2019 ​ 45,511 ​ $ 20.21 ​ ​
Schedule of restricted awards and RSUs	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Restricted ​ Weighted- ​ ​ ​ Stock ​ Average Fair ​ ​ ​ Awards and ​ Value at Grant ​ ​ Units Date ​ Unvested balance, December 31, 2018 ​ 284,879 ​ $ 85.98 ​ Granted ​ 158,608 ​ ​ 100.32 ​ Forfeited ​ (19,865) ​ ​ 93.65 ​ Vested ​ (129,008) ​ ​ 78.81 ​ Unvested balance, September 30, 2019 ​ 294,614 ​ $ 96.32 ​
Schedule of unvested PSAs and their weighted-average fair value at the date of grant	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ ​ ​ ​ Average Fair ​ ​ ​ Performance-Based Restricted Stock Awards ​ Value at Grant ​ ​ Minimum ​ Maximum Target Date Unvested balance, December 31, 2018 ​ 50,823 ​ 173,944 ​ 112,385 ​ $ 96.58 ​ Granted ​ 10,926 ​ 76,480 ​ 43,703 ​ ​ 114.55 ​ Performance adjustment (1) ​ 18,063 ​ (29,819) ​ (5,878) ​ ​ — ​ Forfeited ​ (1,538) ​ (5,604) ​ (3,571) ​ ​ 103.26 ​ Vested ​ (43,650) ​ (43,650) ​ (43,650) ​ ​ 84.78 ​ Unvested balance, September 30, 2019 ​ 34,624 ​ 171,351 ​ 102,989 ​ $ 107.84 ​ (1) Includes the annual adjustment for the number of PSAs earned based on our achievement of relative TSR measured versus the MSCI US REIT Index for the applicable performance periods.
Summary of assumptions used to value stock option awards	​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ 2019 ​ 2018 ​ Expected term (in years) ​ 2.82 ​ 2.82 ​ Expected volatility ​ 24.09 % 24.15 % Expected annual dividend (1) ​ — ​ — ​ Risk-free rate ​ 2.48 % 2.24 % (1) The fair value of the PSAs assumes reinvestment of dividends.

Earnings Per Share (Tables)

Earnings Per Share (Tables)	9 Months Ended
	Sep. 30, 2019
Earnings Per Share
Summary of basic and diluted income per share	​ The following is a summary of basic and diluted net income per share (in thousands, except share and per share amounts): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ ​ 2019 2018 2019 2018 Net income attributable to common shares ​ $ 17,450 ​ $ 18,600 ​ $ 56,646 ​ $ 58,291 ​ Weighted-average common shares outstanding - basic ​ ​ 36,950,950 ​ ​ 35,512,091 ​ ​ 36,589,593 ​ ​ 34,504,790 ​ Effect of potentially dilutive common shares: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Stock options ​ ​ 38,172 ​ ​ 45,453 ​ ​ 38,663 ​ ​ 49,349 ​ Unvested awards ​ ​ 143,033 ​ ​ 163,934 ​ ​ 134,985 ​ ​ 139,696 ​ Weighted-average common shares outstanding - diluted ​ ​ 37,132,155 ​ ​ 35,721,478 ​ ​ 36,763,241 ​ ​ 34,693,835 ​ Net income per share attributable to common shares ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Basic ​ $ 0.47 ​ $ 0.52 ​ $ 1.55 ​ $ 1.69 ​ Diluted ​ $ 0.47 ​ $ 0.52 ​ $ 1.54 ​ $ 1.68 ​

Organization and Description _2

Organization and Description of Business (Details) - Operating partnership	Sep. 30, 2019	Dec. 31, 2018
Coresite Realty Corporation
Organization
Percentage of ownership interests held by the entity	77.60%	75.80%
Noncontrolling interests
Organization
Percentage of ownership interests held by others	22.40%	24.20%
Carlyle Group
Organization
Percentage of ownership interests held by others	22.40%

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Real Estate (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments in Real Estate
Capitalized interest costs	$ 3.7	$ 1.3	$ 10	$ 3.5
Depreciation expense	$ 35.1	$ 31.4	$ 100.1	$ 91.3
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 ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Summary of Significant Accounting Policies
Impairment loss related to intangible assets	$ 0	$ 0	$ 0	$ 0
Goodwill	40,646		40,646		$ 40,646
Impairment loss related to goodwill	0	0	0	0
Deferred Costs
Amortization of initial direct costs	4,000	4,000	10,800	11,800
Internal sales commissions	15,340		15,340		14,199
Third party commissions	11,066		11,066		9,855
Other	485		485		597
Total	26,891		26,891		$ 24,651
Recoverability of Long-Lived Assets
Impairment of long-lived assets	$ 0	$ 0	$ 0	$ 0

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Internal Use Software, RR & IT (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Internal-Use Software
Recognized impairment of internal use software	$ 0	$ 0	$ 0	$ 0
Operating revenues:
Increase to rental revenue			(219)	$ (494)
Allowance for doubtful accounts (in dollars)	214		214		$ 417
Asset Retirement and Environmental Remediation Obligations
Asset retirement and environmental remediation obligations	1,700		$ 1,700		$ 1,600
Income taxes
Minimum percentage of distribution of taxable income to qualify as REIT			90.00%
Percentage of distribution of taxable income as per company policy			100.00%
Provision for federal income taxes			$ 0
Liabilities for uncertain positions	$ 0		$ 0

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Concentration of Credit Risks & Segment Information (Details)	9 Months Ended
	Sep. 30, 2019segment
Segment information
Number of reportable segments	1

Investment in Real Estate (Deta

Investment in Real Estate (Details) $ in Thousands	Apr. 12, 2019USD ($)ft²a	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)
Investment in Real Estate
Land		$ 90,300	$ 86,955
Buildings and Improvements		1,896,820	1,730,329
Construction in Progress		389,174	265,921
Total investments in real estate		2,376,294	2,083,205
Area of land | a	3.8
Purchase price	$ 26,000
Net rentable space (in square feet) | ft²	200,000
Boston
Investment in Real Estate
Land		5,154	5,154
Buildings and Improvements		111,239	107,596
Construction in Progress		5,757	1,644
Total investments in real estate		122,150	114,394
Chicago
Investment in Real Estate
Land		5,493	5,493
Buildings and Improvements		115,265	113,875
Construction in Progress		73,488	14,478
Total investments in real estate		194,246	133,846
Denver
Investment in Real Estate
Buildings and Improvements		32,075	30,740
Construction in Progress		1,477	476
Total investments in real estate		33,552	31,216
Los Angeles
Investment in Real Estate
Land		18,672	18,672
Buildings and Improvements		368,428	338,011
Construction in Progress		52,062	51,688
Total investments in real estate		439,162	408,371
Miami
Investment in Real Estate
Land		728	728
Buildings and Improvements		14,476	14,014
Construction in Progress		124	69
Total investments in real estate		15,328	14,811
New York
Investment in Real Estate
Land		2,729	2,729
Buildings and Improvements		155,012	152,956
Construction in Progress		38,641	33,796
Total investments in real estate		196,382	189,481
Northern Virginia
Investment in Real Estate
Land		21,856	22,793
Buildings and Improvements		397,131	346,209
Construction in Progress		97,242	94,623
Total investments in real estate		516,229	463,625
San Francisco Bay
Investment in Real Estate
Land		35,668	31,386
Buildings and Improvements		703,194	626,928
Construction in Progress		120,383	69,147
Total investments in real estate		$ 859,245	$ 727,461

Other Assets (Details)

Other Assets (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Other Assets.
Deferred rent receivable	$ 38,943	$ 43,162
Initial direct costs	26,891	24,651
Internal-use software	17,033	17,708
Prepaid expense	9,117	7,090
Corporate furniture, fixtures and equipment	4,829	5,006
Deferred financing costs - revolving credit facility	2,241	2,880
Other	3,878	1,793
Total	$ 102,932	$ 102,290

Leases - (Detail)

Leases - (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019USD ($)ft²	Sep. 30, 2018USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2018
Lease terms
Option to extend			20
Weighted-average remaining lease term	9 years		9 years		10 years
Weighted average discount rate - Operating leases	4.90%		4.90%		4.90%
Increase in Right-of-Use Asset	$ 1,000		$ 1,035	$ 114,989
Increase in operating lease liability	1,000
Lease expense
Operating lease expense	6,868	$ 6,144	19,662	16,789
Variable lease expense	1,463	1,185	4,090	3,487
Rent expense	$ 8,331	$ 7,329	$ 23,752	$ 20,276
Minimum
Lease terms
Remaining lease terms	1 year		1 year
Maximum
Lease terms
Remaining lease terms	10 years		10 years
DE1 Denver CO [Member]
Lease terms
Net Leased Space | ft²	5,300

Lease Revenue (Details)

Lease Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Components of lease revenue
Minimum lease revenue	$ 103,518	$ 98,294	$ 304,813	$ 291,488
Variable lease revenue	22,291	23,185	65,605	62,075
Total lease revenue	$ 125,809	$ 121,479	$ 370,418	$ 353,563

Debt - Summary of Debt (Details

Debt - Summary of Debt (Details) - USD ($) $ in Thousands	Apr. 03, 2019	Jan. 31, 2019	Dec. 06, 2018	Sep. 30, 2019	Apr. 17, 2019	Dec. 31, 2018
Debt
Total principal outstanding				$ 1,387,250		$ 1,136,500
Unamortized deferred financing costs				(6,709)		(5,677)
Total debt				$ 1,380,541		$ 1,130,823
Interest Rate Swap | Designated as Hedging Instrument
Debt
Derivative, Fixed Interest Rate			2.91%
Derivative terminated or settled	$ 175,000	$ 50,000
Term		5 years	7 years
Revolving Credit Facility
Debt
Interest rate as of reporting date (as a percent)				3.47%		3.95%
Total principal outstanding				$ 62,250		$ 211,500
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)				3.12%		3.37%
Total principal outstanding				$ 150,000		$ 150,000
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)				3.42%		3.90%
Total principal outstanding				$ 100,000		$ 100,000
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)				3.42%		3.65%
Total principal outstanding				$ 200,000		$ 200,000
Senior Unsecured Term Loan 2023 | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000		$ 150,000
2023 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Interest rate as of reporting date (as a percent)				3.77%		4.01%
Total principal outstanding				$ 150,000
Amount of debt hedged				$ 75,000
Percentage of debt hedged				50.00%
Amount of debt not hedged				$ 75,000
Derivative, Fixed Interest Rate				4.12%
2023 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)				4.19%		4.19%
Total principal outstanding				$ 150,000		$ 150,000
2024 Senior unsecured notes
Debt
Interest rate as of reporting date (as a percent)				3.91%		3.91%
2024 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding				$ 175,000		$ 175,000
2026 Senior unsecured notes
Debt
Interest rate as of reporting date (as a percent)					4.52%
2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)				4.52%
Total principal outstanding				$ 200,000
2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)				4.31%
Total principal outstanding				$ 200,000
Operating partnership | Interest Rate Swap | Designated as Hedging Instrument
Debt
Derivative terminated or settled	$ 175,000
Operating partnership | 2020 Senior Unsecured Term Loan
Debt
Amount of debt not hedged				$ 75,000
Operating partnership | 2020 Senior Unsecured Term Loan | Interest Rate Swap
Debt
Interest rate as of reporting date (as a percent)				3.12%
Amount of debt hedged				$ 75,000
Percentage of debt hedged				50.00%
Derivative, Fixed Interest Rate				2.83%
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Interest rate as of reporting date (as a percent)	4.52%
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Notes | Interest Rate Swap
Debt
Derivative terminated or settled				$ 175,000
Term				7 years

Debt - Revolving Credit Facilit

Debt - Revolving Credit Facility (Details) $ in Thousands	Apr. 19, 2018USD ($)item	Feb. 02, 2016USD ($)	Jun. 24, 2015USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2018USD ($)	Apr. 18, 2018USD ($)	Apr. 19, 2017USD ($)
Debt
Total principal outstanding				$ 1,387,250	$ 1,136,500
Revolving Credit Facility
Debt
Total principal outstanding				$ 62,250	211,500
Credit Agreement
Debt
Leverage ratio (as a percent)				29.30%
Secured debt ratio, Covenant (as a percent)				0.00%
Fixed charge coverage ratio				5.9
Credit Agreement | Minimum
Debt
Fixed charge coverage ratio				1.5
Credit Agreement | Maximum
Debt
Leverage ratio (as a percent)				0.60%
Secured debt ratio (as a percent)				0.40%
Credit Agreement | Senior Unsecured Term Loan
Debt
Maximum borrowing capacity	$ 850,000
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000	150,000
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				100,000	100,000
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				200,000	$ 200,000
2023 Senior unsecured notes
Debt
Term of loan facility	5 years
Unsecured term loan	$ 150,000
2023 Senior unsecured notes | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	1.40%
2023 Senior unsecured notes | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	2.00%
2023 Senior unsecured notes | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	0.40%
2023 Senior unsecured notes | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	1.00%
2023 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000
Operating partnership | LIBOR
Debt
Interest rate margin (as a percent)				1.45%
Operating partnership | Credit Agreement
Debt
Number of options available for extensions | item	1
Extension fee (as a percent)	0.10%
Total increase in commitment under accordion feature	$ 250,000
Maximum borrowing capacity	850,000					$ 600,000
Maximum borrowing capacity including accordion feature	1,200,000
Borrowing capacity at end of period				$ 450,000
Letters of credit outstanding				4,900
Amount available for borrowing under the facility				$ 382,800
Operating partnership | Credit Agreement | Revolving Credit Facility
Debt
Maximum borrowing capacity	450,000					$ 350,000
Operating partnership | Credit Agreement | Revolving Credit Facility | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.45%
Operating partnership | Credit Agreement | Revolving Credit Facility | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.05%
Operating partnership | Credit Agreement | Revolving Credit Facility | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.45%
Operating partnership | Credit Agreement | Revolving Credit Facility | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.05%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Term of loan facility			5 years
Unsecured term loan	$ 150,000		$ 150,000
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Term of loan facility		5 years
Unsecured term loan		$ 100,000
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan							$ 100,000
Increase in accordion feature borrowing capacity							$ 300,000
Operating partnership | 2023 Senior unsecured notes | LIBOR
Debt
Interest rate margin (as a percent)				1.40%

Debt - Unsecured Debt (Details)

Debt - Unsecured Debt (Details) - USD ($) $ in Thousands	Apr. 19, 2018	Feb. 02, 2016	Jun. 24, 2015	Sep. 30, 2019	Jul. 17, 2019	Apr. 17, 2019	Apr. 03, 2019	Dec. 31, 2018	Apr. 20, 2017	Apr. 19, 2017	Jun. 15, 2016
Debt
Total principal outstanding				$ 1,387,250				$ 1,136,500
2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000				$ 150,000
Effective interest rate				3.12%				3.37%
2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 100,000				$ 100,000
Effective interest rate				3.42%				3.90%
2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 200,000				$ 200,000
Effective interest rate				3.42%				3.65%
2023 Senior unsecured notes
Debt
Unsecured term loan	$ 150,000
Term of loan facility	5 years
2023 Senior unsecured notes | LIBOR | Minimum
Debt
Interest rate margin (as a percent)	1.40%
2023 Senior unsecured notes | LIBOR | Maximum
Debt
Interest rate margin (as a percent)	2.00%
2023 Senior unsecured notes | Base Rate | Minimum
Debt
Interest rate margin (as a percent)	0.40%
2023 Senior unsecured notes | Base Rate | Maximum
Debt
Interest rate margin (as a percent)	1.00%
2023 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Total principal outstanding				$ 150,000
Effective interest rate				3.77%				4.01%
Amount of debt not hedged				$ 75,000
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
Debt
Effective interest rate				3.91%				3.91%
2024 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding				$ 175,000				$ 175,000
2026 Senior unsecured notes
Debt
Effective interest rate						4.52%
2026 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Unsecured term loan						$ 200,000
2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding				$ 200,000
Effective interest rate				4.52%
2029 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Unsecured term loan					$ 75,000
2029 Senior unsecured notes | Senior Unsecured Notes
Debt
Total principal outstanding				$ 200,000
Effective interest rate				4.31%
Operating partnership | LIBOR
Debt
Interest rate margin (as a percent)				1.45%
Operating partnership | 2020 Senior Unsecured Term Loan
Debt
Amount of debt not hedged				$ 75,000
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2020 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2020 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2020 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan	$ 150,000		$ 150,000
Term of loan facility			5 years
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2021 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2021 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2021 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan		$ 100,000
Term of loan facility		5 years
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Minimum
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2022 Senior Unsecured Term Loan | LIBOR | Maximum
Debt
Interest rate margin (as a percent)				2.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Minimum
Debt
Interest rate margin (as a percent)				0.40%
Operating partnership | 2022 Senior Unsecured Term Loan | Base Rate | Maximum
Debt
Interest rate margin (as a percent)				1.00%
Operating partnership | 2022 Senior Unsecured Term Loan | Senior Unsecured Term Loan
Debt
Unsecured term loan										$ 100,000
Increase in accordion feature borrowing capacity										300,000
Aggregate principal amount										$ 200,000
Operating partnership | 2023 Senior unsecured notes | LIBOR
Debt
Interest rate margin (as a percent)				1.40%
Operating partnership | 2023 Senior unsecured notes | Senior Unsecured Notes | Private Placement
Debt
Aggregate principal amount											$ 150,000
Stated interest rate											4.19%
Percentage of term notes that may be prepaid											100.00%
Percentage of term notes that holders have right to purchase in cash											100.00%
Percentage of term notes that holders may redeem for cash											100.00%
Operating partnership | 2024 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Unsecured term loan									$ 175,000
Stated interest rate									3.91%
Percentage of term notes that may be prepaid									100.00%
Percentage of term notes that holders have right to purchase in cash									100.00%
Percentage of term notes that holders may redeem for cash									100.00%
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Term Loan
Debt
Stated interest rate						4.11%
Operating partnership | 2026 Senior unsecured notes | Senior Unsecured Notes
Debt
Effective interest rate							4.52%
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 Term Loan
Debt
Aggregate principal amount						$ 125,000
Operating partnership | 2029 Senior unsecured notes | Senior Unsecured Term Loan | Private Placement
Debt
Unsecured term loan						$ 200,000
Stated interest rate						4.31%
Operating partnership | 2029 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%

Debt - Maturities (Details)

Debt - Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt maturities
2020	$ 150,000
2021	100,000
2022	262,250
2023	300,000
Thereafter	575,000
Total principal outstanding	1,387,250	$ 1,136,500
Unamortized deferred financing costs	(6,709)	(5,677)
Total debt	$ 1,380,541	$ 1,130,823

Derivatives and Hedging Activ_3

Derivatives and Hedging Activities (Details) - USD ($) $ in Thousands	Apr. 03, 2019	Jan. 31, 2019	Dec. 06, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2020	Dec. 31, 2018
Derivative instruments and hedging activities disclosures
Unrealized (loss) gain recorded in other comprehensive income				$ (463)	$ 523	$ (5,608)	$ 1,356
Interest expense				10,986	$ 9,433	30,795	$ 26,078
Interest Rate Swap | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount				150,000		150,000			$ 375,000
Strike Rate			2.91%
Derivative terminated or settled	$ 175,000	$ 50,000
Term		5 years	7 years
Amount of income (loss) recorded in accumulated other comprehensive income	(5,700)
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging [Member] | Forecast
Derivative instruments and hedging activities disclosures
Amount estimated to be reclassified as an increase to interest expense								$ 1,500
Interest Rate Swap | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value				(3,281)		(3,281)			(2,938)
Interest rate swap expiring 1/31/2019 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount									50,000
Interest rate swap expiring 1/31/2019 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value									43
Interest rate swap expiring 5/20/2015 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount				$ 75,000		$ 75,000			75,000
Strike Rate				2.83%		2.83%
Interest rate swap expiring 5/20/2015 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value				$ 153		$ 153			1,099
Interest rate swap expiring 5/5/2023 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount				$ 75,000		$ 75,000			75,000
Strike Rate				4.12%		4.12%
Interest rate swap expiring 5/5/2023 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value				$ (3,434)		$ (3,434)			(897)
Interest rate swap expiring 6/3/2026 | Designated as Hedging Instrument
Derivative instruments and hedging activities disclosures
Notional Amount									175,000
Interest rate swap expiring 6/3/2026 | Designated as Hedging Instrument | Level 2
Derivative instruments and hedging activities disclosures
Fair Value									$ (3,183)
Series A Notes | Senior Notes
Derivative instruments and hedging activities disclosures
Debt face amount	$ 200,000
Debt Term	7 years

Stockholders' Equity (Details)

Stockholders' Equity (Details) - Common Stock - $ / shares	Sep. 04, 2019	May 16, 2019	Mar. 07, 2019	Sep. 30, 2019
Stockholders' Equity
Dividends Payable, Date of Record	Sep. 30, 2019	Jun. 28, 2019	Mar. 29, 2019
Common Stock dividend declared (per share)	$ 1.22	$ 1.22	$ 1.10	$ 3.54

Noncontrolling Interests - Op_3

Noncontrolling Interests - Operating Partnership (Details) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Dec. 31, 2018shares
Noncontrolling Interests - Operating Partnership
Redemption value of the noncontrolling interest | $	$ 1,300
Closing price of the Company's stock (in dollars per share) | $ / shares	$ 121.85
Carlyle Group
Noncontrolling Interests - Operating Partnership
Common Operating Partnership units redeemed for common stock (in shares)	803,610
Sale of common stock (in shares)	803,610
Reduction to noncontrolling interests in the Operating Partnership and an increase to common stock and additional paid in capital | $	$ 5.2
Operating partnership
Noncontrolling Interests - Operating Partnership
Conversion ratio for common operating partnership units	1
Number of Operating Partnership Units (in shares)	48,001,664	47,831,300
Percentage of Total	100.00%	100.00%
Shares of common stock issued related to employee compensation arrangements	170,364
Quarterly distributions will receive by holders of operating partnership units (in dollars per unit) | $ / shares	$ 3.54
Operating partnership | Coresite Realty Corporation
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)	37,230,006	36,256,032
Percentage of Total	77.60%	75.80%
Operating partnership | Noncontrolling interests
Noncontrolling Interests - Operating Partnership
Number of Operating Partnership Units (in shares)	10,771,658	11,575,268
Percentage of Total	22.40%	24.20%

Equity Incentive Plan (Details)

Equity Incentive Plan (Details) - 2010 Plan $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2019USD ($)$ / sharesshares	Sep. 30, 2018
Equity Incentive Plan
Shares of common stock reserved for issuance	6,000,000
Shares of common stock that were available for issuance	2,674,453
Stock Options
Number of Shares Subject to Option
Options outstanding at the beginning of the period (in shares)	49,133
Exercised (in shares)	(3,622)
Options outstanding at the end of the period (in shares)	45,511
Weighted Average Exercise Price
Options outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 20.06
Exercised (in dollars per share) | $ / shares	18.10
Options outstanding at the end of the period (in dollars per share) | $ / shares	$ 20.21
Restricted Awards
Compensation cost not yet recognized
Unearned compensation | $	$ 22.1
Restricted Awards | Weighted Average
Compensation cost not yet recognized
Vesting period	2 years 6 months
Restricted awards
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	284,879
Granted (in shares)	158,608
Forfeited (in shares)	(19,865)
Vested (in shares)	(129,008)
Unvested balance at the end of the period (in shares)	294,614
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share) | $ / shares	$ 85.98
Granted (in dollars per share) | $ / shares	100.32
Forfeited (in dollars per share) | $ / shares	93.65
Vested (in dollars per share) | $ / shares	78.81
Unvested balance at the end of the period (in dollars per share) | $ / shares	$ 96.32
PSAs
Compensation cost not yet recognized
Unearned compensation | $	$ 6.5
Assumptions used to value the stock options granted
Expected term	2 years 9 months 25 days	2 years 9 months 25 days
Expected volatility (as a percent)	24.09%	24.15%
Risk-free rate (as a percent)	2.48%	2.24%
Performance period	3 years
PSAs | Weighted Average
Weighted Average Fair Value at Grant
Unvested balance at the beginning of the period (in dollars per share) | $ / shares	$ 96.58
Granted (in dollars per share) | $ / shares	114.55
Performance adjustment (in dollars per share) | $ / shares	103.26
Vested (in dollars per share) | $ / shares	84.78
Unvested balance at the end of the period (in dollars per share) | $ / shares	$ 107.84
Compensation cost not yet recognized
Vesting period	2 years 1 month 6 days
PSAs | Minimum
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	50,823
Granted (in shares)	10,926
Performance adjustment	18,063
Forfeited (in shares)	(1,538)
Vested (in shares)	(43,650)
Unvested balance at the end of the period (in shares)	34,624
Compensation cost not yet recognized
Percentage of target number of shares for PSAs granted in years 2015, 2016 and 2017	25.00%
Assumptions used to value the stock options granted
Performance period	1 year
PSAs | Maximum
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	173,944
Granted (in shares)	76,480
Performance adjustment	(29,819)
Forfeited (in shares)	(5,604)
Vested (in shares)	(43,650)
Unvested balance at the end of the period (in shares)	171,351
Compensation cost not yet recognized
Percentage of target number of shares for PSAs granted in years 2015, 2016 and 2017	175.00%
Assumptions used to value the stock options granted
Performance period	3 years
PSAs | Target
Restricted Awards and PSA's
Unvested balance at the beginning of the period (in shares)	112,385
Granted (in shares)	43,703
Performance adjustment	(5,878)
Forfeited (in shares)	(3,571)
Vested (in shares)	(43,650)
Unvested balance at the end of the period (in shares)	102,989

Earnings Per Share (Details)

Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share
Net income attributable to common shares	$ 17,450	$ 18,600	$ 56,646	$ 58,291
Weighted-average common shares outstanding - basic	36,950,950	35,512,091	36,589,593	34,504,790
Effect of potentially dilutive common shares:
Weighted-average common shares outstanding - diluted	37,132,155	35,721,478	36,763,241	34,693,835
Net income per share attributable to common shares
Basic	$ 0.47	$ 0.52	$ 1.55	$ 1.69
Diluted	$ 0.47	$ 0.52	$ 1.54	$ 1.68
Weighted-average potentially dilutive securities excluded in calculation of basic and diluted income (loss) per share	100	1,003	1,810	121,373
Stock Options
Effect of potentially dilutive common shares:
Potentially dilutive common shares	38,172	45,453	38,663	49,349
Restricted awards
Effect of potentially dilutive common shares:
Potentially dilutive common shares	143,033	163,934	134,985	139,696

Estimated Fair Value of Finan_2

Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Estimated Fair Value of Financial Instruments
Total principal outstanding	$ 1,387,250	$ 1,136,500
Fair value of debt	$ 1,400,000	$ 1,100,000

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ in Millions	Apr. 27, 2018	Sep. 30, 2019	Dec. 31, 2019
Commitments and Contingencies
Estimated reimbursement amount		$ 3.1
Telecommunications Capacity
Commitments and Contingencies
Open commitments		73.8
Telecommunications Capacity | Forecast
Commitments and Contingencies
Open commitments met			$ 3.3
Construction Contracts
Commitments and Contingencies
Open commitments		$ 216.3
Pending Litigation
Commitments and Contingencies
Sought amount	$ 21
Pending Litigation | Damages Related To Construction Project
Commitments and Contingencies
Sought amount	$ 2.8