Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
ASSETS
Real estate and development in process, net		$ 16,234,920	$ 15,925,028
Liabilities and Members'/Partners' Equity [Abstract]
Mortgage notes payable, net		2,986,283	2,063,087
Other Liabilities		452,608	450,821
Total liabilities and equity / capital		19,281,762	18,851,643
Carrying value of the Company's investments in unconsolidated joint ventures		819,368	775,198
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net		1,599,268	1,519,217
Other assets		315,170	297,263
Total assets		1,914,438	1,816,480
Liabilities and Members'/Partners' Equity [Abstract]
Mortgage notes payable, net		863,981	865,665
Other Liabilities		81,047	67,167
Members'/Partners' equity		969,410	883,648
Total liabilities and equity / capital		1,914,438	1,816,480
Company's share of equity		498,789	450,662
Basis differentials	[1]	299,974	302,449
Carrying value of the Company's investments in unconsolidated joint ventures	[2],[3]	798,763	753,111
Unconsolidated Joint Ventures [Member]
Liabilities and Members'/Partners' Equity [Abstract]
Carrying value of the Company's investments in unconsolidated joint ventures		(20,600)	(22,100)
Colorado Center [Member]
Liabilities and Members'/Partners' Equity [Abstract]
Basis differentials		$ 325,900	$ 328,800

[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 June 30, 2017 and December 31, 2016, there was an aggregate basis differential of approximately $325.9 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 $20.6 million and $22.1 million at June 30, 2017 and December 31, 2016, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.

[3] Investments with deficit balances aggregating approximately $20.6 million and $22.1 million at June 30, 2017 and December 31, 2016, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.