Debt (Schedule of Debt) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Debt [Roll Forward]
Deferred costs, Beginning balance			$ (26,110)
Total debt, Beginning balance			8,059,802
Debt Issuances, Net			1,436,571
Repayments, Extinguishment and Assumptions			(1,023,207)
(Accretion) and Amortization			1,134
Deferred costs, Ending balance	$ (23,374)		(23,374)
Total debt, Ending balance	8,474,300		8,474,300
Gain on extinguishment	252	$ 0	252	$ 5,302
Note assumed by the buyer			55,000	$ 24,000
Unamortized premium written-off			1,100
Mortgages and Other Debt [Member]
Debt [Roll Forward]
Total debt, Beginning balance			3,111,985
Debt Issuances, Net			512
Repayments, Extinguishment and Assumptions			(165,207)
(Accretion) and Amortization			(9,218)
Total debt, Ending balance	2,938,072		2,938,072
Mortgage Notes Payable [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance			3,039,882
Net premiums (discount), Beginning balance			59,402
Deferred costs, Beginning balance			(21,020)
Debt Issuances			512
Repayments, Extinguishment and Assumptions			(159,137)
Repayments, Extinguishment and Assumptions, Net Premiums			(1,389)
Repayments, Extinguishment and Assumptions, Deferred costs			339
(Accretion) and Amortization, (Premiums) Discount			(11,141)
(Accretion) and Amortization, Deferred costs			2,016
Outstanding balance, Ending balance	2,881,257		2,881,257
Net premiums (discount), Ending balance	46,872		46,872
Deferred costs, Ending balance	(18,665)		(18,665)
Total debt, Ending balance	2,881,257		2,881,257
Notes repaid prior to maturity			56,400
Unamortized deferred financing costs written-off			300
Other Debt [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance			33,463
Net premiums (discount), Beginning balance			258
Repayments, Extinguishment and Assumptions			(5,020)
(Accretion) and Amortization, (Premiums) Discount			(93)
Outstanding balance, Ending balance	28,443		28,443
Net premiums (discount), Ending balance	165		165
Total debt, Ending balance	28,443		28,443
Corporate Bonds [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance			2,550,000
Net premiums (discount), Beginning balance			(2,745)
Deferred costs, Beginning balance			(10,922)
Total debt, Beginning balance			2,536,333
Debt Issuances			1,000,000
Debt Issuances, Deferred Costs			(16,941)
Debt Issuances, Net			983,059
(Accretion) and Amortization, (Premiums) Discount			393
(Accretion) and Amortization, Deferred costs			2,512
(Accretion) and Amortization			2,905
Outstanding balance, Ending balance	3,550,000		3,550,000
Net premiums (discount), Ending balance	(2,352)		(2,352)
Deferred costs, Ending balance	(25,351)		(25,351)
Total debt, Ending balance	3,522,297		3,522,297
Convertible Debt [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance			1,000,000
Net premiums (discount), Beginning balance			(17,779)
Deferred costs, Beginning balance			(19,327)
Total debt, Beginning balance			962,894
(Accretion) and Amortization, (Premiums) Discount			2,413
(Accretion) and Amortization, Deferred costs			2,752
(Accretion) and Amortization			5,165
Outstanding balance, Ending balance	1,000,000		1,000,000
Net premiums (discount), Ending balance	(15,366)		(15,366)
Deferred costs, Ending balance	(16,575)		(16,575)
Total debt, Ending balance	968,059		968,059
Credit Facility [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance			1,460,000
Deferred costs, Beginning balance			(11,410)
Total debt, Beginning balance			1,448,590
Debt Issuances			453,000
Debt Issuances, Net			453,000
Repayments, Extinguishment and Assumptions			(858,000)
(Accretion) and Amortization, Deferred costs			2,282
(Accretion) and Amortization			2,282
Outstanding balance, Ending balance	1,055,000		1,055,000
Deferred costs, Ending balance	(9,128)		(9,128)
Total debt, Ending balance	$ 1,045,872		1,045,872
2016 Property Dispositions [Member]
Debt [Roll Forward]
Note assumed by the buyer			$ 55,000