Segment Information - Reconciliation of Consolidated Net Income (Loss) to Consolidated NOI (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting [Abstract]
Net income (loss)	$ 73,844		$ 19,046	$ 7,698	$ 21,271	$ (12,370)	$ 7,884	$ (1,323)	$ 46,310	$ 121,859	$ 40,501	$ (30,740)
Plus:
Interest expense										81,886	84,708	110,717
Interest expense - amortization of deferred financing costs										2,435	2,696	4,557
Interest expense - financing obligation										0	679	1,237
Depreciation and amortization										179,357	189,676	219,029
General and administrative expenses										28,538	26,596	29,406
Equity in loss of Real Estate Ventures										8,306	11,503	811
Provision for impairment								$ 1,800		3,057	40,517	82,208
Loss on early extinguishment of debt										3,933	66,590	0
Less:
Interest income										1,113	1,236	1,224
Income tax benefit										628	0	0
Tax credit transaction income										0	0	19,955
Net gain from remeasurement of investments in real estate ventures										0	0	758
Net gain on disposition of real estate										31,657	116,983	20,496
Net gain on sale of undepreciated real estate										953	9,232	3,019
Net gain on Real Estate Venture transactions										80,526	20,000	7,229
Consolidated net operating income										$ 314,494	$ 316,015	$ 364,544

[1] The increase in net income for the fourth quarter primarily relates to the gain of $52.2 million from the disposition of eight office properties by the Austin Venture. For further details, see Note 4, "Investment in Unconsolidated Real Estate Ventures."