Schedule III - Real Estate Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 51,156
Initial costs of buildings and improvements	364,686
Initial costs of construction in progress	208,608
Costs capitalized subsequent to acquisition, Land	54,385
Costs capitalized subsequent to acquisition, Buildings and improvements	1,552,565
Costs capitalized subsequent to acquisition, Construction in progress	581,455
Gross carrying amount, Land	105,541
Gross carrying amount, Buildings and improvements	1,917,251
Gross carrying amount, Construction in progress	790,064
Accumulated depreciation and amortization	(467,644)	$ (394,823)	$ (317,834)	$ (239,936)
Owned Properties
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	51,156
Initial costs of buildings and improvements	303,614
Initial costs of construction in progress	208,608
Costs capitalized subsequent to acquisition, Land	54,385
Costs capitalized subsequent to acquisition, Buildings and improvements	1,569,440
Costs capitalized subsequent to acquisition, Construction in progress	572,099
Gross carrying amount, Land	105,541
Gross carrying amount, Buildings and improvements	1,873,054
Gross carrying amount, Construction in progress	780,707
Accumulated depreciation and amortization	(436,700)
Owned Properties | Atlanta, Georgia (Atlanta-Metro) [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	12,647
Initial costs of buildings and improvements	35,473
Costs capitalized subsequent to acquisition, Land	7,769
Costs capitalized subsequent to acquisition, Buildings and improvements	457,973
Costs capitalized subsequent to acquisition, Construction in progress	88,253
Gross carrying amount, Land	20,416
Gross carrying amount, Buildings and improvements	493,446
Gross carrying amount, Construction in progress	88,253
Accumulated depreciation and amortization	$ (162,808)
Date of acquisition	Oct. 03, 2006
Owned Properties | Irving Texas
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of buildings and improvements	$ 5,808
Costs capitalized subsequent to acquisition, Land	8,606
Costs capitalized subsequent to acquisition, Buildings and improvements	339,807
Costs capitalized subsequent to acquisition, Construction in progress	99,445
Gross carrying amount, Land	8,606
Gross carrying amount, Buildings and improvements	345,615
Gross carrying amount, Construction in progress	99,445
Accumulated depreciation and amortization	$ (40,650)
Date of acquisition	Feb. 08, 2013
Owned Properties | Richmond, Virginia
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 2,000
Initial costs of buildings and improvements	11,200
Costs capitalized subsequent to acquisition, Land	180
Costs capitalized subsequent to acquisition, Buildings and improvements	241,898
Costs capitalized subsequent to acquisition, Construction in progress	67,932
Gross carrying amount, Land	2,180
Gross carrying amount, Buildings and improvements	253,098
Gross carrying amount, Construction in progress	67,932
Accumulated depreciation and amortization	$ (60,731)
Date of acquisition	Mar. 20, 2010
Owned Properties | Chicago, Illinois
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of construction in progress	$ 17,764
Costs capitalized subsequent to acquisition, Land	9,400
Costs capitalized subsequent to acquisition, Buildings and improvements	130,150
Costs capitalized subsequent to acquisition, Construction in progress	115,331
Gross carrying amount, Land	9,400
Gross carrying amount, Buildings and improvements	130,150
Gross carrying amount, Construction in progress	133,095
Accumulated depreciation and amortization	$ (10,302)
Date of acquisition	Jul. 08, 2014
Owned Properties | Suwanee, Georgia [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 1,395
Initial costs of buildings and improvements	29,802
Costs capitalized subsequent to acquisition, Land	2,126
Costs capitalized subsequent to acquisition, Buildings and improvements	136,496
Costs capitalized subsequent to acquisition, Construction in progress	3,188
Gross carrying amount, Land	3,521
Gross carrying amount, Buildings and improvements	166,298
Gross carrying amount, Construction in progress	3,188
Accumulated depreciation and amortization	$ (66,678)
Date of acquisition	Sep. 01, 2005
Owned Properties | Piscataway New Jersey
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 7,466
Initial costs of buildings and improvements	80,366
Initial costs of construction in progress	13,900
Costs capitalized subsequent to acquisition, Buildings and improvements	17,440
Costs capitalized subsequent to acquisition, Construction in progress	19,572
Gross carrying amount, Land	7,466
Gross carrying amount, Buildings and improvements	97,806
Gross carrying amount, Construction in progress	33,472
Accumulated depreciation and amortization	$ (6,585)
Date of acquisition	Jun. 06, 2016
Owned Properties | Santa Clara, California
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of buildings and improvements	$ 15,838
Costs capitalized subsequent to acquisition, Buildings and improvements	82,710
Costs capitalized subsequent to acquisition, Construction in progress	7,600
Gross carrying amount, Buildings and improvements	98,548
Gross carrying amount, Construction in progress	7,600
Accumulated depreciation and amortization	$ (40,466)
Date of acquisition	Nov. 01, 2007
Owned Properties | Sacramento, California
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 1,481
Initial costs of buildings and improvements	52,753
Costs capitalized subsequent to acquisition, Buildings and improvements	12,121
Costs capitalized subsequent to acquisition, Construction in progress	92
Gross carrying amount, Land	1,481
Gross carrying amount, Buildings and improvements	64,874
Gross carrying amount, Construction in progress	92
Accumulated depreciation and amortization	$ (11,522)
Date of acquisition	Dec. 21, 2012
Owned Properties | Fort Worth, Texas
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 136
Initial costs of buildings and improvements	610
Initial costs of construction in progress	48,984
Costs capitalized subsequent to acquisition, Land	8,943
Costs capitalized subsequent to acquisition, Buildings and improvements	18,013
Costs capitalized subsequent to acquisition, Construction in progress	(5,269)
Gross carrying amount, Land	9,079
Gross carrying amount, Buildings and improvements	18,623
Gross carrying amount, Construction in progress	43,715
Accumulated depreciation and amortization	$ (1,195)
Date of acquisition	Dec. 16, 2016
Owned Properties | Princeton, New Jersey
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 20,700
Initial costs of buildings and improvements	32,126
Costs capitalized subsequent to acquisition, Buildings and improvements	1,920
Costs capitalized subsequent to acquisition, Construction in progress	431
Gross carrying amount, Land	20,700
Gross carrying amount, Buildings and improvements	34,046
Gross carrying amount, Construction in progress	431
Accumulated depreciation and amortization	$ (3,972)
Date of acquisition	Jun. 30, 2014
Owned Properties | Dulles, Virginia
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 3,154
Initial costs of buildings and improvements	29,583
Costs capitalized subsequent to acquisition, Buildings and improvements	42,852
Costs capitalized subsequent to acquisition, Construction in progress	3,852
Gross carrying amount, Land	3,154
Gross carrying amount, Buildings and improvements	72,435
Gross carrying amount, Construction in progress	3,852
Accumulated depreciation and amortization	$ (18,586)
Date of acquisition	Oct. 06, 2017
Owned Properties | Ashburn, Virginia
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of construction in progress	$ 53,009
Costs capitalized subsequent to acquisition, Land	17,325
Costs capitalized subsequent to acquisition, Buildings and improvements	63,245
Costs capitalized subsequent to acquisition, Construction in progress	131,942
Gross carrying amount, Land	17,325
Gross carrying amount, Buildings and improvements	63,245
Gross carrying amount, Construction in progress	184,951
Accumulated depreciation and amortization	$ (1,704)
Date of acquisition	Aug. 09, 2017
Owned Properties | Phoenix, Arizona
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of construction in progress	$ 24,668
Costs capitalized subsequent to acquisition, Construction in progress	4,894
Gross carrying amount, Construction in progress	$ 29,562
Date of acquisition	Aug. 11, 2017
Owned Properties | Hillsboro, Oregon
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of construction in progress	$ 25,657
Costs capitalized subsequent to acquisition, Construction in progress	14,178
Gross carrying amount, Construction in progress	$ 39,835
Date of acquisition	Oct. 03, 2017
Owned Properties | Miami, Florida [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 1,777
Initial costs of buildings and improvements	6,955
Costs capitalized subsequent to acquisition, Buildings and improvements	24,080
Costs capitalized subsequent to acquisition, Construction in progress	90
Gross carrying amount, Land	1,777
Gross carrying amount, Buildings and improvements	31,035
Gross carrying amount, Construction in progress	90
Accumulated depreciation and amortization	$ (11,044)
Date of acquisition	Mar. 06, 2008
Owned Properties | Lenexa, Kansas [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 400
Initial costs of buildings and improvements	3,100
Costs capitalized subsequent to acquisition, Land	36
Costs capitalized subsequent to acquisition, Buildings and improvements	735
Gross carrying amount, Land	436
Gross carrying amount, Buildings and improvements	3,835
Accumulated depreciation and amortization	$ (457)
Date of acquisition	Jun. 03, 2011
Owned Properties | Manassas, Virginia
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of construction in progress	$ 24,626
Costs capitalized subsequent to acquisition, Construction in progress	20,568
Gross carrying amount, Construction in progress	$ 45,194
Date of acquisition	Mar. 02, 2018
Leased Properties
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of buildings and improvements	$ 61,072
Costs capitalized subsequent to acquisition, Buildings and improvements	(16,875)
Costs capitalized subsequent to acquisition, Construction in progress	9,356
Gross carrying amount, Buildings and improvements	44,197
Gross carrying amount, Construction in progress	9,356
Accumulated depreciation and amortization	(30,944)
Leased Properties | Leased Facilities Acquired In 2015 Member
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of buildings and improvements	59,087
Costs capitalized subsequent to acquisition, Buildings and improvements	(37,471)
Costs capitalized subsequent to acquisition, Construction in progress	382
Gross carrying amount, Buildings and improvements	21,616
Gross carrying amount, Construction in progress	382
Accumulated depreciation and amortization	$ (18,586)
Date of acquisition	Jun. 16, 2015
Leased Properties | Jersey City, New Jersey [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of buildings and improvements	$ 1,985
Costs capitalized subsequent to acquisition, Buildings and improvements	19,745
Costs capitalized subsequent to acquisition, Construction in progress	8,952
Gross carrying amount, Buildings and improvements	21,730
Gross carrying amount, Construction in progress	8,952
Accumulated depreciation and amortization	$ (11,757)
Date of acquisition	Nov. 01, 2006
Leased Properties | Overland Park, Kansas [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Costs capitalized subsequent to acquisition, Buildings and improvements	$ 851
Costs capitalized subsequent to acquisition, Construction in progress	22
Gross carrying amount, Buildings and improvements	851
Gross carrying amount, Construction in progress	22
Accumulated depreciation and amortization	$ (601)