Investment in Unconsolidated Joint Ventures - Income Statement of Unconsolidated Joint Ventures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Equity Method Investments [Line Items]
Operating expenses	$ 106,275	$ 73,466	$ 303,824	$ 202,885
Depreciation and amortization			131,742	87,604
Operating income (loss)	9,472	(141)	26,496	11,228
Interest expense	16,543	11,520	48,920	33,011
Loan cost amortization			2,551	1,841
Equity Method Investee
Schedule of Equity Method Investments [Line Items]
Revenues	28,792	299	83,391	299
Operating expenses	11,856	0	33,393	0
Depreciation and amortization	10,601	193	33,102	193
Operating income (loss)	6,335	106	16,896	106
Interest expense	9,920	0	29,254	0
Loan cost amortization	(126)	0	(342)	0
Other expenses	0	0	1	0
Net income (loss)	(3,459)	106	(12,017)	106
Equity Method Investee | US Airways Building
Schedule of Equity Method Investments [Line Items]
Revenues	1,126	299	3,378	299
Operating expenses	0	0	0	0
Depreciation and amortization	522	193	1,566	193
Operating income (loss)	604	106	1,812	106
Interest expense	105	0	312	0
Loan cost amortization	0	0	0	0
Other expenses	0	0	0	0
Net income (loss)	499	106	1,500	106
Equity Method Investee | 7000 Central Park JV LLC
Schedule of Equity Method Investments [Line Items]
Revenues	1,803	0	5,572	0
Operating expenses	944	0	2,817	0
Depreciation and amortization	1,023	0	3,316	0
Operating income (loss)	(164)	0	(561)	0
Interest expense	150	0	213	0
Loan cost amortization	35	0	141	0
Other expenses	0	0	0	0
Net income (loss)	(349)	0	(915)	0
Equity Method Investee | Austin Joint Venture
Schedule of Equity Method Investments [Line Items]
Revenues	25,863	0	74,441	0
Operating expenses	10,912	0	30,576	0
Depreciation and amortization	9,056	0	28,220	0
Operating income (loss)	5,895	0	15,645	0
Interest expense	9,665	0	28,729	0
Loan cost amortization	(161)	0	(483)	0
Other expenses	0	0	1	0
Net income (loss)	$ (3,609)	$ 0	$ (12,602)	$ 0