Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) - USD ($) $ in Thousands		Dec. 31, 2018	Dec. 31, 2017
ASSETS
Real estate and development in process, net		$ 16,752,119	$ 16,507,008
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage and notes payable, net		2,964,572	2,979,281
Other Liabilities		503,726	443,980
Total liabilities and equity / capital		20,256,477	19,372,233
Carying value of the Company's investment in unconsolidated joint ventures		956,309	619,925
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net		3,545,906	1,768,996
Other assets		543,512	367,743
Total assets		4,089,418	2,136,739
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage and notes payable, net		2,017,609	1,437,440
Other Liabilities		582,006	99,215
Members'/Partners' equity		1,489,803	600,084
Total liabilities and equity / capital		4,089,418	2,136,739
Company's share of equity		622,498	286,495
Basis differentials	[1]	308,721	307,485
Carying value of the Company's investment in unconsolidated joint ventures	[2]	931,219	593,980
Colorado Center [Member]
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Basis differentials		316,700	322,500
Unconsolidated Joint Ventures [Member]
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Other Liabilities		$ 25,100	$ 25,900

[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, 2018 and 2017, there was an aggregate basis differential of approximately $316.7 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 $25.1 million and $25.9 million at December 31, 2018 and 2017, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.