Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Summary of financial instruments
Bank line of credit	$ 810,313	$ 397,432
Senior unsecured notes	8,623,376	9,120,107
Mortgage debt	895,289	932,212
Other debt	95,229	94,445
Carrying Value
Summary of financial instruments
Loans receivable, net	768,896	768,743
Marketable debt securities	100,915	102,958
Marketable equity securities	24	39
Bank line of credit	810,313	397,432
Term loans	511,737	524,807
Senior unsecured notes	8,623,376	9,120,107
Mortgage debt	895,289	932,212
Other debt	95,229	94,445
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	16	55
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets		196
Derivative liabilities	(7,384)	6,251
Carrying Value | Currency Swap
Summary of financial instruments
Derivative assets	2,469	1,551
Fair Value | Level 1
Summary of financial instruments
Marketable equity securities	24	39
Senior unsecured notes	8,854,121	9,390,668
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	775,109	770,052
Marketable debt securities	100,915	102,958
Bank line of credit	810,313	397,432
Term loans	511,737	524,807
Mortgage debt	936,466	963,786
Other debt	95,229	94,445
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	16	55
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative assets		196
Derivative liabilities	7,384	6,251
Fair Value | Currency Swap
Summary of financial instruments
Derivative assets	$ 2,469	$ 1,551