Impairment Loss on Real Estate Assets (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jul. 31, 2016 property
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss on real estate assets	$ 0	$ 22,951	$ 0	$ 33,901
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	0	0	8,258
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	22,951	0	22,951
Number of real estate properties sold | property					2
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	$ 0	$ 0	2,692
Impairment loss				2,700
Disposal Group, Disposed of by Sale, Not Discontinued Operations | 9200 and 9211 Corporate Boulevard
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment loss				$ 23,000