Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Summary of financial instruments
Bank line of credit	$ 1,022,324	$ 838,516
Senior unsecured notes	8,567,293	7,626,194
Mortgage debt	967,072	984,431
Other debt	95,144	97,022
Carrying Value
Summary of financial instruments
Loans receivable, net	862,621	906,961
Marketable debt securities	191,846	231,442
Marketable equity securities	46	43
Bank line of credit	1,022,324	838,516
Term loan	561,525	213,610
Senior unsecured notes	8,567,293	7,626,194
Mortgage debt	967,072	984,431
Other debt	95,144	97,022
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	2,190	2,220
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets	1,524	178
Derivative liabilities	7,015	7,663
Carrying Value | Currency Swap
Summary of financial instruments
Derivative assets	303	929
Derivative liabilities	1,570
Fair Value
Summary of financial instruments
Marketable equity securities		43
Bank line of credit	1,022,324	838,516
Fair Value | Level 1
Summary of financial instruments
Marketable debt securities	191,846	252,125
Marketable equity securities	46
Senior unsecured notes	8,873,387	8,187,458
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	866,528	898,522
Term loan	561,525	213,610
Mortgage debt	1,008,930	1,025,091
Other debt	95,144	97,022
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	2,190	2,220
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative assets	1,524	178
Derivative liabilities	7,015	7,663
Fair Value | Currency Swap
Summary of financial instruments
Derivative assets	303	$ 929
Derivative liabilities	$ 1,570