Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Summary of financial instruments
Bank line of credit	$ 1,000,824	$ 838,516
Senior unsecured notes	8,568,729	7,626,194
Mortgage debt	939,982	984,431
Other debt	94,561	97,022
Carrying Value
Summary of financial instruments
Loans receivable, net	765,593	906,961
Marketable debt securities	118,019	231,442
Marketable equity securities	1,254	43
Bank line of credit	1,000,824	838,516
Term loan	540,784	213,610
Senior unsecured notes	8,568,729	7,626,194
Mortgage debt	939,982	984,431
Other debt	94,561	97,022
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	43	2,220
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets		178
Derivative liabilities	7,655	7,663
Carrying Value | Currency Swap
Summary of financial instruments
Derivative assets	1,118	929
Fair Value
Summary of financial instruments
Marketable equity securities		43
Bank line of credit	1,000,824	838,516
Fair Value | Level 1
Summary of financial instruments
Marketable debt securities	118,019	252,125
Marketable equity securities	1,254
Senior unsecured notes	8,865,860	8,187,458
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	779,525	898,522
Term loan	540,784	213,610
Mortgage debt	984,286	1,025,091
Other debt	94,561	97,022
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	43	2,220
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative assets		178
Derivative liabilities	7,655	7,663
Fair Value | Currency Swap
Summary of financial instruments
Derivative assets	$ 1,118	$ 929