Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Summary of financial instruments
Bank line of credit	$ 492,421	$ 899,718
Senior unsecured notes	7,136,336	7,133,538
Mortgage debt	147,329	623,792
Other debt	91,263	92,385
Carrying Value
Summary of financial instruments
Loans receivable, net	788,486	807,954
Marketable debt securities	18,330	68,630
Marketable equity securities	85	76
Bank line of credit	492,421	899,718
Term loans	274,103	440,062
Senior unsecured notes	7,136,336	7,133,538
Mortgage debt	147,329	623,792
Other debt	91,263	92,385
Carrying Value | Warrants
Summary of financial instruments
Derivative assets	39	19
Carrying Value | Interest-rate swap contracts
Summary of financial instruments
Derivative liabilities	4,163	4,857
Carrying Value | Currency Swap | Level 2
Summary of financial instruments
Derivative assets	1,924	2,920
Fair Value | Level 1
Summary of financial instruments
Marketable equity securities	85	76
Senior unsecured notes	7,445,519	7,386,149
Fair Value | Level 2
Summary of financial instruments
Loans receivable, net	787,067	807,505
Marketable debt securities	18,330	68,630
Bank line of credit	492,421	899,718
Term loans	274,103	440,062
Mortgage debt	134,605	609,374
Other debt	91,263	92,385
Fair Value | Warrants | Level 3
Summary of financial instruments
Derivative assets	39	19
Fair Value | Interest-rate swap contracts | Level 2
Summary of financial instruments
Derivative liabilities	4,163	4,857
Fair Value | Currency Swap | Level 2
Summary of financial instruments
Derivative assets	$ 1,924	$ 2,920