Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
ASSETS
Real estate and development in process, net	$ 15,458,406		$ 11,577,979
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage notes payable	4,468,069		3,102,485
Other liabilities	567,464		324,613
Total liabilities and capital	19,151,515		15,081,132
Carrying value of the Company's investments in unconsolidated joint ventures	129,038		659,916
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net	927,890		4,494,971
Other assets	158,105		673,716
Total assets	1,085,995		5,168,687
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage notes payable	748,489		3,039,922
Other liabilities	21,183		792,888
Members'/Partners' equity	316,323		1,335,877
Total liabilities and capital	1,085,995		5,168,687
Company's share of equity	157,875		787,941
Basis differentials	(28,837)	[1]	(128,025)	[1]
Carrying value of the Company's investments in unconsolidated joint ventures	$ 129,038		$ 659,916

[1] unt 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.