Investment in Unconsolidated Joint Ventures - Balance Sheet of Unconsolidated Joint Ventures (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Equity Method Investee	Jun. 30, 2013 Equity Method Investee	Jun. 30, 2014 Equity Method Investee US Airways Building	Jun. 30, 2013 Equity Method Investee US Airways Building	Jun. 30, 2014 Equity Method Investee 7000 Central Park JV LLC	Jun. 30, 2013 Equity Method Investee 7000 Central Park JV LLC	Jun. 30, 2014 Equity Method Investee Austin Joint Venture	Jun. 30, 2013 Equity Method Investee Austin Joint Venture
Schedule of Equity Method Investments [Line Items]
Cash			$ 12,113	$ 0	$ 148	$ 0	$ 1,468	$ 0	$ 10,497	$ 0
Restricted cash			685	0	0	0	170	0	515	0
Real estate, net	2,508,323	2,316,795	812,960	49,945	48,404	49,945	49,060	0	715,496	0
Intangible assets, net			65,033	5,878	5,350	5,878	5,888	0	53,795	0
Other assets			19,145	547	597	547	1,133	0	17,415	0
Total assets	3,131,521	2,925,501	909,936	56,370	54,499	56,370	57,719	0	797,718	0
Mortgage debt	1,111,386	1,097,493	671,353	13,945	13,609	13,945	30,000	0	627,744	0
Other Liabilities			46,399	247	371	247	2,402	0	43,626	0
Partners' equity			192,184	42,178	40,519	42,178	25,317	0	126,348	0
Total liabilities and partners' equity	$ 3,131,521	$ 2,925,501	$ 909,936	$ 56,370	$ 54,499	$ 56,370	$ 57,719	$ 0	$ 797,718	$ 0