Debt (Schedule of Debt) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2015 USD ($)
Debt [Roll Forward]
Total debt, Beginning balance	$ 10,513,781
Debt Issuances	1,379
Repayments, Extinguishment and Assumptions	(1,534,068)
Accretion and (Amortization)	(14,096)
Total debt, Ending balance	8,966,996
Mortgages and Other Debt [Member]
Debt [Roll Forward]
Total debt, Beginning balance	3,805,761
Debt Issuances	1,379
Repayments, Extinguishment and Assumptions	(460,068)
Accretion and (Amortization)	(18,166)
Total debt, Ending balance	3,328,906
Mortgage Notes Payable [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance	3,689,796
Net premiums (Discount), Beginning balance	70,139
Debt Issuances	1,379
Repayments, Extinguishment and Assumptions	(457,900)
Repayments, Extinguishment and Assumptions, Net Premium	8,020
Accretion and (Amortization)	(17,977)
Outstanding balance, Ending balance	3,233,275
Net premiums (Discount), Ending balance	60,182
Total debt, Ending balance	3,233,275
Other Debt [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance	45,325
Net premiums (Discount), Beginning balance	501
Repayments, Extinguishment and Assumptions	(10,188)
Accretion and (Amortization)	(189)
Outstanding balance, Ending balance	35,137
Net premiums (Discount), Ending balance	312
Total debt, Ending balance	35,137
Corporate Bonds [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance	2,550,000
Net premiums (Discount), Beginning balance	(3,501)
Total debt, Beginning balance	2,546,499
Accretion and (Amortization)	560
Outstanding balance, Ending balance	2,550,000
Net premiums (Discount), Ending balance	(2,941)
Total debt, Ending balance	2,547,059
Convertible Debt [Member]
Debt [Roll Forward]
Outstanding balance, Beginning balance	1,000,000
Net premiums (Discount), Beginning balance	(22,479)
Total debt, Beginning balance	977,521
Accretion and (Amortization)	3,510
Outstanding balance, Ending balance	1,000,000
Net premiums (Discount), Ending balance	(18,969)
Total debt, Ending balance	981,031
Credit Facility [Member]
Debt [Roll Forward]
Total debt, Beginning balance	3,184,000
Debt Issuances	0
Repayments, Extinguishment and Assumptions	(1,074,000)
Total debt, Ending balance	2,110,000
Mortgages Secured by Properties Held for Sale [Member]
Debt [Roll Forward]
Outstanding balance, Ending balance	124,300
Net premiums (Discount), Ending balance	$ (5,800)