Impairment Loss on Real Estate Assets (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jul. 31, 2016 property
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	$ 46,461	$ 33,901	$ 43,301
Impairment of real estate			43,301
Disposal Group, Held-for-sale, Not Discontinued Operations | Eastpoint I & II
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	0	0	6,195
Disposal Group, Held-for-sale, Not Discontinued Operations | 2 Gatehall Drive
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	0	0	37,106
Disposal Group, Held-for-sale, Not Discontinued Operations | 150 West Jefferson
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	0	8,259	0
Disposal Group, Held-for-sale, Not Discontinued Operations | 9200 and 9211 Corporate Boulevard
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	0	22,950	0
Number of assets sold | property				2
Impairment of real estate		23,000
Disposal Group, Held-for-sale, Not Discontinued Operations | Disposal Group of 13 Assets
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	46,461	0	0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | 9221 Corporate Boulevard
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	$ 0	2,692	$ 0
Impairment of real estate		$ 2,700