Investment in Unconsolidated Joint Ventures - Income Statement of Unconsolidated Joint Ventures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Equity Method Investments [Line Items]
Operating expenses	$ 101,717	$ 106,275	$ 324,563	$ 303,824
Depreciation and amortization			142,810	131,742
Operating income	13,126	9,472	36,869	26,496
Interest expense	17,017	16,543	53,891	48,920
Loan cost amortization			2,360	2,551
Equity Method Investee
Schedule of Equity Method Investments [Line Items]
Revenues	3,242	28,792	9,376	83,391
Operating expenses	950	11,856	2,933	33,393
Depreciation and amortization	1,444	10,601	5,258	33,102
Operating income	848	6,335	1,185	16,896
Interest expense	270	9,920	771	29,254
Loan cost amortization	624	(126)	708	(342)
Other expenses				1
Net income (loss)	(46)	(3,459)	(294)	(12,017)
Equity Method Investee | US Airways Building
Schedule of Equity Method Investments [Line Items]
Revenues	1,126	1,126	3,378	3,378
Operating expenses	3	0	3	0
Depreciation and amortization	522	522	1,566	1,566
Operating income	601	604	1,809	1,812
Interest expense	101	105	304	312
Loan cost amortization	0	0	0	0
Other expenses				0
Net income (loss)	500	499	1,505	1,500
Equity Method Investee | 7000 Central Park
Schedule of Equity Method Investments [Line Items]
Revenues	2,116	1,803	5,998	5,572
Operating expenses	947	944	2,930	2,817
Depreciation and amortization	922	1,023	3,692	3,316
Operating income	247	(164)	(624)	(561)
Interest expense	169	150	467	213
Loan cost amortization	624	35	708	141
Other expenses				0
Net income (loss)	(546)	(349)	(1,799)	(915)
Equity Method Investee | Austin Joint Venture
Schedule of Equity Method Investments [Line Items]
Revenues	0	25,863	0	74,441
Operating expenses	0	10,912	0	30,576
Depreciation and amortization	0	9,056	0	28,220
Operating income	0	5,895	0	15,645
Interest expense	0	9,665	0	28,729
Loan cost amortization	0	(161)	0	(483)
Other expenses				1
Net income (loss)	$ 0	$ (3,609)	$ 0	$ (12,602)