Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Summary of financial instruments
Bank line of credit	$ 899,718	$ 397,432
Senior unsecured notes	7,133,538	9,120,107
Mortgage debt	623,792	932,212
Other debt	92,385	94,445
Carrying Value
Summary of financial instruments
Loans receivable, net	807,954	768,743
Marketable debt securities	68,630	102,958
Marketable equity securities	76	39
Bank line of credit	899,718	397,432
Term loans	440,062	524,807
Senior unsecured notes	7,133,538	9,120,107
Mortgage debt	623,792	932,212
Other debt	92,385	94,445
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	19	55
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative assets		196
Derivative liabilities	4,857	6,251
Carrying Value | Currency Swap
Summary of financial instruments
Derivative assets	2,920	1,551
Fair Value | Level 1
Summary of financial instruments
Marketable debt securities	68,630	102,958
Marketable equity securities	76	39
Senior unsecured notes	7,386,149	9,390,668
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	807,505	770,052
Bank line of credit	899,718	397,432
Term loans	440,062	524,807
Mortgage debt	609,374	963,786
Other debt	92,385	94,445
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	19	55
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative assets		196
Derivative liabilities	4,857	6,251
Fair Value | Currency Swap | Level 2
Summary of financial instruments
Derivative assets	$ 2,920	$ 1,551