Schedule III - Real Estate Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	183,982
Costs capitalized subsequent to acquisition, Land	37,060
Costs capitalized subsequent to acquisition, Buildings and improvements	1,336,601
Costs capitalized subsequent to acquisition, Construction in progress	383,837
Gross carrying amount, Land	88,216
Gross carrying amount, Buildings and improvements	1,701,287
Gross carrying amount, Construction in progress	567,819
Accumulated depreciation and amortization	(394,823)	$ (317,834)	$ (239,936)	$ (180,167)
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	183,982
Costs capitalized subsequent to acquisition, Land	37,060
Costs capitalized subsequent to acquisition, Buildings and improvements	1,337,486
Costs capitalized subsequent to acquisition, Construction in progress	378,265
Gross carrying amount, Land	88,216
Gross carrying amount, Buildings and improvements	1,641,100
Gross carrying amount, Construction in progress	562,247
Accumulated depreciation and amortization	(358,327)
Owned Properties | Atlanta, Georgia (Atlanta-Metro)
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	417,363
Costs capitalized subsequent to acquisition, Construction in progress	28,614
Gross carrying amount, Land	20,416
Gross carrying amount, Buildings and improvements	452,836
Gross carrying amount, Construction in progress	28,614
Accumulated depreciation and amortization	$ (139,859)
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	271,086
Costs capitalized subsequent to acquisition, Construction in progress	86,320
Gross carrying amount, Land	8,606
Gross carrying amount, Buildings and improvements	276,894
Gross carrying amount, Construction in progress	86,320
Accumulated depreciation and amortization	$ (23,997)
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	243,403
Costs capitalized subsequent to acquisition, Construction in progress	61,888
Gross carrying amount, Land	2,180
Gross carrying amount, Buildings and improvements	254,603
Gross carrying amount, Construction in progress	61,888
Accumulated depreciation and amortization	$ (49,561)
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	81,463
Costs capitalized subsequent to acquisition, Construction in progress	117,715
Gross carrying amount, Land	9,400
Gross carrying amount, Buildings and improvements	81,463
Gross carrying amount, Construction in progress	135,479
Accumulated depreciation and amortization	$ (4,619)
Date of acquisition	Jul. 08, 2014
Owned Properties | Suwanee, Georgia
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	136,113
Costs capitalized subsequent to acquisition, Construction in progress	3,620
Gross carrying amount, Land	3,521
Gross carrying amount, Buildings and improvements	165,915
Gross carrying amount, Construction in progress	3,620
Accumulated depreciation and amortization	$ (61,029)
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	2,885
Costs capitalized subsequent to acquisition, Construction in progress	23,907
Gross carrying amount, Land	7,466
Gross carrying amount, Buildings and improvements	83,251
Gross carrying amount, Construction in progress	37,807
Accumulated depreciation and amortization	$ (3,610)
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	84,190
Costs capitalized subsequent to acquisition, Construction in progress	6,989
Gross carrying amount, Buildings and improvements	100,028
Gross carrying amount, Construction in progress	6,989
Accumulated depreciation and amortization	$ (36,662)
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	11,498
Costs capitalized subsequent to acquisition, Construction in progress	58
Gross carrying amount, Land	1,481
Gross carrying amount, Buildings and improvements	64,251
Gross carrying amount, Construction in progress	58
Accumulated depreciation and amortization	$ (9,224)
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	17,284
Costs capitalized subsequent to acquisition, Construction in progress	(15,210)
Gross carrying amount, Land	9,079
Gross carrying amount, Buildings and improvements	17,894
Gross carrying amount, Construction in progress	33,774
Accumulated depreciation and amortization	$ (434)
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	822
Costs capitalized subsequent to acquisition, Construction in progress	451
Gross carrying amount, Land	20,700
Gross carrying amount, Buildings and improvements	32,948
Gross carrying amount, Construction in progress	451
Accumulated depreciation and amortization	$ (3,059)
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	46,656
Costs capitalized subsequent to acquisition, Construction in progress	3,565
Gross carrying amount, Land	3,154
Gross carrying amount, Buildings and improvements	76,239
Gross carrying amount, Construction in progress	3,565
Accumulated depreciation and amortization	$ (15,861)
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, Construction in progress	53,943
Gross carrying amount, Construction in progress	$ 106,952
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	2,734
Gross carrying amount, Construction in progress	$ 27,402
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	3,621
Gross carrying amount, Construction in progress	$ 29,278
Date of acquisition	Oct. 03, 2017
Owned Properties | Miami, Florida
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	23,989
Costs capitalized subsequent to acquisition, Construction in progress	48
Gross carrying amount, Land	1,777
Gross carrying amount, Buildings and improvements	30,944
Gross carrying amount, Construction in progress	48
Accumulated depreciation and amortization	$ (10,077)
Date of acquisition	Mar. 06, 2008
Owned Properties | Lenexa, Kansas
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	734
Costs capitalized subsequent to acquisition, Construction in progress	2
Gross carrying amount, Land	436
Gross carrying amount, Buildings and improvements	3,834
Gross carrying amount, Construction in progress	2
Accumulated depreciation and amortization	$ (335)
Date of acquisition	Jun. 03, 2011
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	(885)
Costs capitalized subsequent to acquisition, Construction in progress	5,572
Gross carrying amount, Buildings and improvements	60,187
Gross carrying amount, Construction in progress	5,572
Accumulated depreciation and amortization	(36,496)
Leased Properties | Leased Facilities acquired in 2015
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	(23,278)
Costs capitalized subsequent to acquisition, Construction in progress	146
Gross carrying amount, Buildings and improvements	35,809
Gross carrying amount, Construction in progress	146
Accumulated depreciation and amortization	$ (24,512)
Date of acquisition	Jun. 16, 2015
Leased Properties | Jersey City, New Jersey
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	21,663
Costs capitalized subsequent to acquisition, Construction in progress	5,388
Gross carrying amount, Buildings and improvements	23,648
Gross carrying amount, Construction in progress	5,388
Accumulated depreciation and amortization	$ (11,411)
Date of acquisition	Nov. 01, 2006
Leased Properties | Overland Park, Kansas
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Costs capitalized subsequent to acquisition, Buildings and improvements	$ 730
Costs capitalized subsequent to acquisition, Construction in progress	38
Gross carrying amount, Buildings and improvements	730
Gross carrying amount, Construction in progress	38
Accumulated depreciation and amortization	$ (573)