Schedule III - Real Estate Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	$ 43,429
Initial costs of buildings and improvements	275,857
Initial costs of contruction in progress	29,891
Costs capitalized subsequent to acquisition, Land	13,683
Costs capitalized subsequent to acquisition, Buildings and improvements	904,529
Costs capitalized subsequent to acquisition, Construction in progress	315,764
Gross carrying amount, Land	57,112
Gross carrying amount, Buildings and improvements	1,180,386
Gross carrying amount, Construction in progress	345,655
Accumulated depreciation and amortization	(239,936)	$ (180,167)	$ (137,725)	$ (102,900)
Owned Properties [Member]
SEC Schedule III, Real Estate and Accumulated Depreciation [Line Items]
Initial costs of land	42,299
Initial costs of buildings and improvements	194,975
Initial costs of contruction in progress	17,764
Costs capitalized subsequent to acquisition, Land	13,683
Costs capitalized subsequent to acquisition, Buildings and improvements	866,272
Costs capitalized subsequent to acquisition, Construction in progress	318,270
Gross carrying amount, Land	55,982
Gross carrying amount, Buildings and improvements	1,061,247
Gross carrying amount, Construction in progress	336,034
Accumulated depreciation and amortization	(217,253)
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	120,226
Costs capitalized subsequent to acquisition, Construction in progress	15,330
Gross carrying amount, Land	3,521
Gross carrying amount, Buildings and improvements	150,028
Gross carrying amount, Construction in progress	15,330
Accumulated depreciation and amortization	$ (50,038)
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	370,717
Costs capitalized subsequent to acquisition, Construction in progress	41,835
Gross carrying amount, Land	15,397
Gross carrying amount, Buildings and improvements	406,190
Gross carrying amount, Construction in progress	41,835
Accumulated depreciation and amortization	$ (95,856)
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	78,599
Costs capitalized subsequent to acquisition, Construction in progress	1,379
Gross carrying amount, Buildings and improvements	94,437
Gross carrying amount, Construction in progress	1,379
Accumulated depreciation and amortization	$ (28,524)
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	197,454
Costs capitalized subsequent to acquisition, Construction in progress	85,771
Gross carrying amount, Land	2,180
Gross carrying amount, Buildings and improvements	208,654
Gross carrying amount, Construction in progress	85,771
Accumulated depreciation and amortization	$ (23,918)
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	8,709
Costs capitalized subsequent to acquisition, Construction in progress	73
Gross carrying amount, Land	1,481
Gross carrying amount, Buildings and improvements	61,462
Gross carrying amount, Construction in progress	73
Accumulated depreciation and amortization	$ (4,956)
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, Buildings and improvements	582
Costs capitalized subsequent to acquisition, Construction in progress	422
Gross carrying amount, Land	20,700
Gross carrying amount, Buildings and improvements	32,708
Gross carrying amount, Construction in progress	422
Accumulated depreciation and amortization	$ (1,284)
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	8,590
Costs capitalized subsequent to acquisition, Buildings and improvements	65,975
Costs capitalized subsequent to acquisition, Construction in progress	120,331
Gross carrying amount, Land	8,590
Gross carrying amount, Buildings and improvements	71,783
Gross carrying amount, Construction in progress	120,331
Accumulated depreciation and amortization	$ (4,468)
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 contruction in progress	$ 17,764
Costs capitalized subsequent to acquisition, Construction in progress	52,985
Gross carrying amount, Construction in progress	$ 70,749
Date of acquisition	Jul. 08, 2014
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	23,599
Costs capitalized subsequent to acquisition, Construction in progress	144
Gross carrying amount, Land	1,777
Gross carrying amount, Buildings and improvements	30,554
Gross carrying amount, Construction in progress	144
Accumulated depreciation and amortization	$ (8,101)
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	411
Gross carrying amount, Land	437
Gross carrying amount, Buildings and improvements	3,511
Accumulated depreciation and amortization	$ (108)
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
Gross carrying amount, Land	1,899
Gross carrying amount, Buildings and improvements	$ 1,920
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 contruction in progress	12,127
Costs capitalized subsequent to acquisition, Buildings and improvements	38,257
Costs capitalized subsequent to acquisition, Construction in progress	(2,506)
Gross carrying amount, Land	1,130
Gross carrying amount, Buildings and improvements	119,139
Gross carrying amount, Construction in progress	9,621
Accumulated depreciation and amortization	(22,683)
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 contruction in progress	12,127
Costs capitalized subsequent to acquisition, Buildings and improvements	11,092
Costs capitalized subsequent to acquisition, Construction in progress	(4,931)
Gross carrying amount, Land	1,130
Gross carrying amount, Buildings and improvements	89,989
Gross carrying amount, Construction in progress	7,196
Accumulated depreciation and amortization	$ (8,758)
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	26,243
Costs capitalized subsequent to acquisition, Construction in progress	2,421
Gross carrying amount, Buildings and improvements	28,228
Gross carrying amount, Construction in progress	2,421
Accumulated depreciation and amortization	$ (13,273)
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	$ 922
Costs capitalized subsequent to acquisition, Construction in progress	4
Gross carrying amount, Buildings and improvements	922
Gross carrying amount, Construction in progress	4
Accumulated depreciation and amortization	$ (652)