Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
ASSETS
Real estate and development in process, net		$ 16,641,806	$ 16,507,008
Liabilities and Members'/Partners' Equity [Abstract]
Mortgage notes payable, net		2,974,930	2,979,281
Other Liabilities		470,140	443,980
Total liabilities and equity / capital		19,583,939	19,372,233
Carrying value of the Company's investments in unconsolidated joint ventures		666,718	619,925
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net		1,844,695	1,768,996
Other assets		376,127	367,743
Total assets		2,220,822	2,136,739
Liabilities and Members'/Partners' Equity [Abstract]
Mortgage notes payable, net		1,471,762	1,437,440
Other Liabilities		95,597	99,215
Members'/Partners' equity		653,463	600,084
Total liabilities and equity / capital		2,220,822	2,136,739
Company's share of equity		335,580	286,495
Basis differentials	[1]	305,936	307,485
Carrying value of the Company's investments in unconsolidated joint ventures	[2],[3]	641,516	593,980
Unconsolidated Joint Ventures [Member]
Liabilities and Members'/Partners' Equity [Abstract]
Other Liabilities		(25,200)	(25,900)
Basis differentials		$ 321,100	$ 322,500

[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 result from impairments of investments, acquisitions through joint ventures with no change in control 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. At March 31, 2018 and December 31, 2017, there was an aggregate basis differential of approximately $321.1 million and $322.5 million, respectively, between the carrying value of the Company’s investment in the joint venture that owns Colorado Center and the joint venture’s basis in the assets and liabilities, which differential (excluding land) shall be amortized over the remaining lives of the related assets and liabilities.

[2] Investments with deficit balances aggregating approximately $25.2 million and $25.9 million at March 31, 2018 and December 31, 2017, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.

[3] Investments with deficit balances aggregating approximately $25.2 million and $25.9 million at March 31, 2018 and December 31, 2017, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.