Schedule III - Real Estate Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 51,031
Initial costs of buildings and improvements	356,833
Initial costs of construction in progress	92,775
Costs capitalized subsequent to acquisition, Land	23,099
Costs capitalized subsequent to acquisition, Buildings and improvements	1,167,933
Costs capitalized subsequent to acquisition, Construction in progress	273,185
Gross carrying amount, Land	74,130
Gross carrying amount, Buildings and improvements	1,524,767
Gross carrying amount, Construction in progress	365,960
Accumulated depreciation and amortization	(317,834)	$ (239,936)	$ (180,167)	$ (137,725)
Owned Properties [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	49,901
Initial costs of buildings and improvements	275,951
Initial costs of construction in progress	80,648
Costs capitalized subsequent to acquisition, Land	23,099
Costs capitalized subsequent to acquisition, Buildings and improvements	1,131,563
Costs capitalized subsequent to acquisition, Construction in progress	274,781
Gross carrying amount, Land	73,000
Gross carrying amount, Buildings and improvements	1,407,515
Gross carrying amount, Construction in progress	355,429
Accumulated depreciation and amortization	(280,294)
Owned Properties [Member] | 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	141,574
Costs capitalized subsequent to acquisition, Construction in progress	2,013
Gross carrying amount, Land	3,521
Gross carrying amount, Buildings and improvements	171,376
Gross carrying amount, Construction in progress	2,013
Accumulated depreciation and amortization	$ (59,326)
Date of acquisition	Sep. 01, 2005
Owned Properties [Member] | 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	2,750
Costs capitalized subsequent to acquisition, Buildings and improvements	399,492
Costs capitalized subsequent to acquisition, Construction in progress	32,422
Gross carrying amount, Land	15,397
Gross carrying amount, Buildings and improvements	434,965
Gross carrying amount, Construction in progress	32,422
Accumulated depreciation and amortization	$ (118,470)
Date of acquisition	Oct. 03, 2006
Owned Properties [Member] | Santa Clara, California [Member]
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	82,870
Costs capitalized subsequent to acquisition, Construction in progress	7,078
Gross carrying amount, Buildings and improvements	98,708
Gross carrying amount, Construction in progress	7,078
Accumulated depreciation and amortization	$ (33,701)
Date of acquisition	Nov. 01, 2007
Owned Properties [Member] | Richmond, Virginia [Member]
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	226,147
Costs capitalized subsequent to acquisition, Construction in progress	70,580
Gross carrying amount, Land	2,180
Gross carrying amount, Buildings and improvements	237,347
Gross carrying amount, Construction in progress	70,580
Accumulated depreciation and amortization	$ (36,376)
Date of acquisition	Mar. 20, 2010
Owned Properties [Member] | Sacramento, California [Member]
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	9,349
Costs capitalized subsequent to acquisition, Construction in progress	390
Gross carrying amount, Land	1,481
Gross carrying amount, Buildings and improvements	62,102
Gross carrying amount, Construction in progress	390
Accumulated depreciation and amortization	$ (7,079)
Date of acquisition	Dec. 21, 2012
Owned Properties [Member] | Princeton, New Jersey [Member]
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, Land	0
Costs capitalized subsequent to acquisition, Buildings and improvements	662
Costs capitalized subsequent to acquisition, Construction in progress	538
Gross carrying amount, Land	20,700
Gross carrying amount, Buildings and improvements	32,788
Gross carrying amount, Construction in progress	538
Accumulated depreciation and amortization	$ (2,169)
Date of acquisition	Jun. 30, 2014
Owned Properties [Member] | Dallas-Fort Worth, Texas [Member]
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	198,905
Costs capitalized subsequent to acquisition, Construction in progress	69,653
Gross carrying amount, Land	8,606
Gross carrying amount, Buildings and improvements	204,713
Gross carrying amount, Construction in progress	69,653
Accumulated depreciation and amortization	$ (11,465)
Date of acquisition	Feb. 08, 2013
Owned Properties [Member] | Chicago, Illinois [Member]
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	45,848
Costs capitalized subsequent to acquisition, Construction in progress	82,859
Gross carrying amount, Land	9,400
Gross carrying amount, Buildings and improvements	45,848
Gross carrying amount, Construction in progress	100,623
Accumulated depreciation and amortization	$ (853)
Date of acquisition	Jul. 08, 2014
Owned Properties [Member] | Piscataway New Jersey [Member]
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, Land	0
Costs capitalized subsequent to acquisition, Buildings and improvements	1,844
Costs capitalized subsequent to acquisition, Construction in progress	3,361
Gross carrying amount, Land	7,466
Gross carrying amount, Buildings and improvements	82,210
Gross carrying amount, Construction in progress	17,261
Accumulated depreciation and amortization	$ (1,240)
Date of acquisition	Jun. 06, 2016
Owned Properties [Member] | Fort Worth, Texas [Member]
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	0
Costs capitalized subsequent to acquisition, Buildings and improvements	0
Costs capitalized subsequent to acquisition, Construction in progress	132
Gross carrying amount, Land	136
Gross carrying amount, Buildings and improvements	610
Gross carrying amount, Construction in progress	49,116
Accumulated depreciation and amortization	$ (8)
Date of acquisition	Dec. 16, 2016
Owned Properties [Member] | 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	24,215
Costs capitalized subsequent to acquisition, Construction in progress	83
Gross carrying amount, Land	1,777
Gross carrying amount, Buildings and improvements	31,170
Gross carrying amount, Construction in progress	83
Accumulated depreciation and amortization	$ (9,390)
Date of acquisition	Mar. 06, 2008
Owned Properties [Member] | 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	37
Costs capitalized subsequent to acquisition, Buildings and improvements	660
Gross carrying amount, Land	437
Gross carrying amount, Buildings and improvements	3,760
Accumulated depreciation and amortization	$ (214)
Date of acquisition	Jun. 03, 2011
Owned Properties [Member] | Duluth Georgia Office Building [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 1,899
Initial costs of buildings and improvements	1,920
Costs capitalized subsequent to acquisition, Buildings and improvements	(2)
Costs capitalized subsequent to acquisition, Construction in progress	5,672
Gross carrying amount, Land	1,899
Gross carrying amount, Buildings and improvements	1,918
Gross carrying amount, Construction in progress	5,672
Accumulated depreciation and amortization	$ (3)
Date of acquisition	Dec. 30, 2015
Leased Properties [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 1,130
Initial costs of buildings and improvements	80,882
Initial costs of construction in progress	12,127
Costs capitalized subsequent to acquisition, Buildings and improvements	36,369
Costs capitalized subsequent to acquisition, Construction in progress	(1,596)
Gross carrying amount, Land	1,130
Gross carrying amount, Buildings and improvements	117,252
Gross carrying amount, Construction in progress	10,531
Accumulated depreciation and amortization	(37,540)
Leased Properties [Member] | Carpathia Properties [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	1,130
Initial costs of buildings and improvements	78,897
Initial costs of construction in progress	12,127
Costs capitalized subsequent to acquisition, Buildings and improvements	13,670
Costs capitalized subsequent to acquisition, Construction in progress	(4,456)
Gross carrying amount, Land	1,130
Gross carrying amount, Buildings and improvements	92,567
Gross carrying amount, Construction in progress	7,671
Accumulated depreciation and amortization	$ (26,956)
Date of acquisition	Jun. 16, 2015
Leased Properties [Member] | 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	21,738
Costs capitalized subsequent to acquisition, Construction in progress	2,332
Gross carrying amount, Buildings and improvements	23,723
Gross carrying amount, Construction in progress	2,332
Accumulated depreciation and amortization	$ (9,896)
Date of acquisition	Nov. 01, 2006
Leased Properties [Member] | Overland Park, Kansas [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Costs capitalized subsequent to acquisition, Buildings and improvements	$ 962
Costs capitalized subsequent to acquisition, Construction in progress	528
Gross carrying amount, Buildings and improvements	962
Gross carrying amount, Construction in progress	528
Accumulated depreciation and amortization	$ (687)