Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2015

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]

Schedule of Fair Values of Financial Assets and Liabilities by Levels [Table Text Block]

(Amounts in thousands)		As of December 31, 2015
		Total		Level 1		Level 2		Level 3
Marketable securities		$	150,997	$	150,997	$	-	$	-
Real estate fund investments (75% of which is attributable to
	noncontrolling interests)		574,761		-		-		574,761
Deferred compensation plan assets (included in other assets)			117,475		58,289		-		59,186
	Total assets	$	843,233	$	209,286	$	-	$	633,947
Mandatorily redeemable instruments (included in other liabilities)		$	50,561	$	50,561	$	-	$	-
Interest rate swaps (included in other liabilities)			19,600		-		19,600		-
	Total liabilities	$	70,161	$	50,561	$	19,600	$	-
(Amounts in thousands)		As of December 31, 2014
		Total		Level 1		Level 2		Level 3
Marketable securities		$	206,323	$	206,323	$	-	$	-
Real estate fund investments (75% of which is attributable to
	noncontrolling interests)		513,973		-		-		513,973
Deferred compensation plan assets (included in other assets)			117,284		53,969		-		63,315
	Total assets	$	837,580	$	260,292	$	-	$	577,288
Mandatorily redeemable instruments (included in other liabilities)		$	55,097	$	55,097	$	-	$	-
Interest rate swap (included in other liabilities)			25,797		-		25,797		-
	Total liabilities	$	80,894	$	55,097	$	25,797	$	-

Non-financial Assets Measured at Fair Value on a Nonrecurring Basis [Table Text Block]

(Amounts in thousands)	As of December 31, 2015
	Total		Level 1		Level 2		Level 3
Real estate assets	$	-	$	-	$	-	$	-
(Amounts in thousands)	As of December 31, 2014
	Total		Level 1		Level 2		Level 3
Real estate assets	$	4,848	$	-	$	-	$	4,848

Schedule Of Carrying Amounts And Fair Values Of Financial Instruments [Table Text Block]

(Amounts in thousands)			As of December 31, 2015				As of December 31, 2014
			Carrying		Fair		Carrying		Fair
			Amount		Value		Amount		Value
Cash equivalents			$	1,295,980	$	1,296,000	$	749,418	$	749,000
Mezzanine loan receivable (included in other assets)				-		-		16,748		17,000
			$	1,295,980	$	1,296,000	$	766,166	$	766,000
Debt:
	Mortgages payable		$	9,614,838	$	9,306,000	$	8,261,055	$	8,224,000
	Senior unsecured notes			850,000		868,000		1,350,000		1,385,000
	Unsecured term loan			187,500		187,500		-		-
	Unsecured revolving credit facilities			550,000		550,000		-		-
		Total	$	11,202,338	$	10,911,500	$	9,611,055	$	9,609,000

Real Estate Fund [Member]

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]

Fair Value Inputs, Assets, Quantitative Information [Table Text Block]

			Weighted Average
			(based on fair
Unobservable Quantitative Input		Range	value of investments)
	Discount rates	12.0% to 14.9%	13.6%
	Terminal capitalization rates	4.8% to 6.1%	5.5%

Summary of Changes in Level 3 Plan Assets [Table Text Block]

(Amounts in thousands)	For The Year Ended December 31,
	2015		2014
Beginning balance	$	513,973	$	667,710
Purchases		95,010		3,392
Dispositions / Distributions		(91,450)		(307,268)
Net unrealized gains		54,995		73,802
Net realized gains		2,757		76,337
Other, net		(524)		-
Ending balance	$	574,761	$	513,973

Deferred Compensation Plan Assets [Member]

Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]

Summary of Changes in Level 3 Plan Assets [Table Text Block]

(Amounts in thousands)	For The Year Ended December 31,
	2015		2014
Beginning balance	$	63,315	$	68,782
Purchases		9,062		14,162
Sales		(13,252)		(24,951)
Realized and unrealized gains		(501)		3,415
Other, net		562		1,907
Ending balance	$	59,186	$	63,315