Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Summary of financial instruments
Bank line of credit	$ 136,311	$ 899,718
Senior unsecured notes	6,889,045	7,133,538
Mortgage debt	146,337	623,792
Other debt	94,801	92,385
Carrying Value
Summary of financial instruments
Loans receivable, net	393,575	807,954
Marketable debt securities	18,448	68,630
Marketable equity securities	82	76
Bank line of credit	136,311	899,718
Term loans	218,832	440,062
Senior unsecured notes	6,889,045	7,133,538
Mortgage debt	146,337	623,792
Other debt	94,801	92,385
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	61	19
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative liabilities	3,272	4,857
Carrying Value | Currency swap
Summary of financial instruments
Derivative assets	0	2,920
Derivative liabilities	5,504	0
Fair Value | Level 1
Summary of financial instruments
Marketable equity securities	82	76
Senior unsecured notes	7,267,553	7,386,149
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	394,438	807,505
Marketable debt securities	18,448	68,630
Bank line of credit	136,311	899,718
Term loans	218,832	440,062
Mortgage debt	133,494	609,374
Other debt	94,801	92,385
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	61	19
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative liabilities	3,272	4,857
Fair Value | Currency swap | Level 2
Summary of financial instruments
Derivative assets	0	2,920
Derivative liabilities	$ 5,504	$ 0