Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) - USD ($) $ in Thousands		Mar. 31, 2019		Jan. 01, 2019	Dec. 31, 2018
ASSETS
Real estate and development in process, net		$ 17,116,764			$ 16,752,119
Liabilities and Members'/Partners' Equity [Abstract]
Mortgage notes payable, net		2,959,908			2,964,572
Other Liabilities		369,575			503,726
Total liabilities and equity / capital		20,501,604			20,256,477
Carrying value of the Company's investments in unconsolidated joint ventures		976,580			956,309
Right of use assets - finance leases		187,292			0
Right of use assets - operating leases		151,166		$ 151,800	0
Lease liabilities - finance leases		173,123			0
Lease liabilities - operating leases		199,653		$ 199,300	0
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net		3,660,439	[1]		3,545,906
Other assets		534,534			543,512
Total assets		4,194,973			4,089,418
Liabilities and Members'/Partners' Equity [Abstract]
Mortgage notes payable, net		2,074,473			2,017,609
Other Liabilities		604,280	[2]		582,006
Members'/Partners' equity		1,516,220			1,489,803
Total liabilities and equity / capital		4,194,973			4,089,418
Company's share of equity		642,564			622,498
Basis differentials	[3]	310,072			308,721
Carrying value of the Company's investments in unconsolidated joint ventures	[4],[5]	952,636			931,219
Unconsolidated Joint Ventures [Member]
Liabilities and Members'/Partners' Equity [Abstract]
Other Liabilities		(23,900)			(25,100)
Basis differentials		315,300			$ 316,700
Right of use assets - finance leases		248,900
Right of use assets - operating leases		12,700
Lease liabilities - finance leases		393,900
Lease liabilities - operating leases		$ 17,200

[1] (1)At March 31, 2019, this amount includes right of use assets - finance leases and right of use assets - operating leases totaling approximately $248.9 million and $12.7 million, respectively.

[2] At March 31, 2019, this amount includes lease liabilities - finance leases and lease liabilities - operating leases totaling approximately $393.9 million and $17.2 million, respectively.

[3] 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, 2019 and December 31, 2018, there was an aggregate basis differential of approximately $315.3 million and $316.7 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.

[4] Investments with deficit balances aggregating approximately $23.9 million and $25.1 million at March 31, 2019 and December 31, 2018, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheet

[5] Investments with deficit balances aggregating approximately $23.9 million and $25.1 million at March 31, 2019 and December 31, 2018, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.