Investments in Unconsolidated Joint Ventures (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Assets
Commercial real estate property, net	$ 10,270,331		$ 10,269,630
Other assets	722,952		718,326
Liabilities and members' equity
Mortgages and other loans payable	4,815,485		4,615,464
Company's net investment in unconsolidated joint ventures	1,073,130		1,032,243
Combined statements of income for the unconsolidated joint ventures
Operating expense	73,633	73,254
Ground rent	10,990	8,806
Real estate taxes	53,688	51,480
Interest expense, net of interest income	81,336	80,137
Depreciation and amortization	85,146	80,663
Transaction related costs	1,358	1,056
Total expenses	327,218	316,142
Company's equity in net income (loss) of unconsolidated joint ventures	5,073	(1,560)
Joint venture
Assets
Commercial real estate property, net	6,965,366		6,910,991
Other assets	789,684		728,113
Total assets	7,755,050		7,639,104
Liabilities and members' equity
Mortgages and other loans payable	5,364,303		5,358,048
Other liabilities	405,820		406,929
Members' equity	1,984,927		1,874,127
Total liabilities and members equity	7,755,050		7,639,104
Company's net investment in unconsolidated joint ventures	1,073,130		1,032,243
Combined statements of income for the unconsolidated joint ventures
Total revenues	151,231	120,048
Operating expense	28,611	16,766
Ground rent	657	918
Real estate taxes	17,305	13,374
Interest expense, net of interest income	56,407	54,868
Depreciation and amortization	46,894	38,550
Transaction related costs		268
Total expenses	149,874	124,744
Net (loss) income before gain on sale	1,357	(4,696)
Company's equity in net income (loss) of unconsolidated joint ventures	$ 5,073	$ (1,560)