Investments in Unconsolidated Joint Ventures - Balance Sheet Footnotes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Investments in unconsolidated joint ventures:
Total assets	$ 3,541,130		$ 3,541,130		$ 3,176,604
Mortgage notes payable that could become recourse debt to the Company	5,000		5,000		33,540
Indemnity of guaranteed amount	2,500		2,500		16,770
Amortization of difference between cost of investments and book value of underlying equity	3,348	$ 948	3,188	$ 3,227
Northwestern Mutual Life (NML)
Investments in unconsolidated joint ventures:
Mortgage notes payable to affiliate	463,821		463,821		606,263
Interest expense on borrowings from related party	6,385	$ 9,645	22,976	$ 28,992
Tysons Corner LLC
Investments in unconsolidated joint ventures:
Total assets	278,100		278,100		341,931
Total liabilities	$ 836,659		$ 836,659		$ 871,933