Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Schedule of Equity Method Investments [Line Items]
Real estate and development in process, net	$ 11,959,168		$ 10,746,486
Mortgage and Notes payable	3,102,485		3,123,267
Other liabilities	324,613		293,515
Total liabilities and equity	15,462,321		14,782,966
Carrying value of the Company's investments in unconsolidated joint ventures	659,916		669,722
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Real estate and development in process, net	4,494,971		4,542,594
Other assets	673,716		668,113
Total assets	5,168,687		5,210,707
Mortgage and Notes payable	3,039,922		2,988,894
Other liabilities	792,888		854,257
Members'/Partners' equity	1,335,877		1,367,556
Total liabilities and equity	5,168,687		5,210,707
Company's share of equity	787,941		799,479
Basis differentials	(128,025)	[1]	(129,757)	[1]
Carrying value of the Company's investments in unconsolidated joint ventures	$ 659,916		$ 669,722

[1] This amount represents the aggregate difference between the Company’s historical cost basis and the basis reflected at the joint venture level, which is typically amortized over the life of the related assets and liabilities. Basis differentials occur from impairment of investments and upon the transfer of assets that were previously owned by the Company into a joint venture. In addition, certain acquisition, transaction and other costs may not be reflected in the net assets at the joint venture level.