Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Summary of financial instruments
Bank line of credit	$ 605,837	$ 899,718
Senior unsecured notes	6,393,926	7,133,538
Mortgage debt	145,417	623,792
Other debt	94,818	92,385
Carrying Value
Summary of financial instruments
Loans receivable, net	402,152	807,954
Marketable debt securities	18,567	68,630
Marketable equity securities	74	76
Bank line of credit	605,837	899,718
Term loans	226,205	440,062
Senior unsecured notes	6,393,926	7,133,538
Mortgage debt	145,417	623,792
Other debt	94,818	92,385
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	58	19
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative liabilities	2,980	4,857
Carrying Value | Currency swap
Summary of financial instruments
Derivative assets	0	2,920
Derivative liabilities	9,469	0
Fair Value | Level 1
Summary of financial instruments
Marketable equity securities	74	76
Senior unsecured notes	6,769,010	7,386,149
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	402,267	807,505
Marketable debt securities	18,567	68,630
Bank line of credit	605,837	899,718
Term loans	226,205	440,062
Mortgage debt	131,419	609,374
Other debt	94,818	92,385
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	58	19
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative liabilities	2,980	4,857
Fair Value | Currency swap | Level 2
Summary of financial instruments
Derivative assets	0	2,920
Derivative liabilities	$ 9,469	$ 0