Real Estate (Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition) (Details) - USD ($) $ in Thousands				12 Months Ended
		May. 31, 2013	Apr. 10, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restructuring Cost and Reserve [Line Items]
Real estate and related intangibles recorded upon consolidation				$ 0	$ 0	$ 3,356,000
Mortgage notes payable				(3,438,714)	(4,309,484)
Mezzanine notes payable				(308,482)	(309,796)
Outside members' notes payable				(180,000)	(180,000)
Debt recorded upon consolidation				0	0	(2,056,000)
Cash and cash equivalents					0	79,468
Accrued interest payable				(190,386)	(163,532)
Working capital recorded upon consolidation				0	0	(177,315)
Noncontrolling interest in property partnerships recorded upon consolidation				$ 0	$ 0	$ (480,861)
Mountain View Research And Technology Parks [Member]
Restructuring Cost and Reserve [Line Items]
Land			$ 126,521
Building and improvements			82,451
Tenant Improvements			7,326
In-place lease Intangibles			23,279
Above market rents			843
Below market rents			(7,336)
Aggregate purchase price			$ 233,084
767 Fifth Avenue (the General Motors Building)
Restructuring Cost and Reserve [Line Items]
Land		$ 1,796,252
Building and improvements		1,447,446
Tenant Improvements		85,208
In-place lease Intangibles		357,781
Above market rents		101,897
Below market rents		(239,641)
Above Market Assumed Debt Adjustments		(192,943)
Real estate and related intangibles recorded upon consolidation		3,356,000
Mortgage notes payable		(1,300,000)
Mezzanine notes payable		(306,000)
Outside members' notes payable	[1]	(450,000)
Debt recorded upon consolidation		(2,056,000)
Cash and cash equivalents		79,468
Cash held in escrows		2,403
Tenant and other receivables		7,104
Prepaid expenses and other assets		4,269
Accounts payable and accrued expenses		(2,418)
Accrued interest payable	[2]	(182,369)
Other liabilities		(6,304)
Working capital recorded upon consolidation		(97,847)
Noncontrolling Interest		(520,000)
Noncontrolling Interest working capital		39,139
Noncontrolling interest in property partnerships recorded upon consolidation		(480,861)
Net Assets Recorded Upon Consolidation		721,292
Consolidation, Eliminations [Member] | 767 Fifth Avenue (the General Motors Building)
Restructuring Cost and Reserve [Line Items]
Outside members' notes payable		(270,000)
Accrued interest payable		$ (105,500)

[1] The Company’s member loan totaling $270.0 million eliminates in consolidation.

[2] The Company’s share of the accrued interest payable on the members’ loans totaling approximately $105.5 million eliminates in consolidation.