Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
ASSETS
Real estate and development in process, net		$ 16,507,008	$ 15,925,028
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage and notes payable, net		2,979,281	2,063,087
Other Liabilities		443,980	450,821
Total liabilities and equity / capital		19,372,233	18,851,643
Carying value of the Company's investment in unconsolidated joint ventures		619,925	775,198
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net		1,768,996	1,519,217
Other assets		367,743	297,263
Total assets		2,136,739	1,816,480
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage and notes payable, net		1,437,440	865,665
Other Liabilities		99,215	67,167
Members'/Partners' equity		600,084	883,648
Total liabilities and equity / capital		2,136,739	1,816,480
Company's share of equity		286,495	450,662
Basis differentials	[1]	307,485	302,449
Carying value of the Company's investment in unconsolidated joint ventures	[2]	593,980	753,111
Colorado Center [Member]
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Basis differentials		322,500	328,800
Unconsolidated Joint Ventures [Member]
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Other Liabilities		$ 25,900	$ 22,100

[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 December 31, 2017 and 2016, there was an aggregate basis differential of approximately $322.5 million and $328.8 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 $25.9 million and $22.1 million at December 31, 2017 and 2016, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.