Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Summary of financial instruments
Bank line of credit	$ 869,078	$ 397,432
Senior unsecured notes	8,626,559	9,120,107
Mortgage debt	688,910	932,212
Other debt	93,012	94,445
Carrying Value
Summary of financial instruments
Loans receivable, net	708,096	768,743
Marketable debt securities	84,128	102,958
Marketable equity securities	34	39
Bank line of credit	869,078	397,432
Term loans	477,890	524,807
Senior unsecured notes	8,626,559	9,120,107
Mortgage debt	688,910	932,212
Other debt	93,012	94,445
Level 1 | Fair Value
Summary of financial instruments
Marketable equity securities	34	39
Senior unsecured notes	9,098,192	9,390,668
Level 2 | Fair Value
Summary of financial instruments
Loans receivable, net	715,450	770,052
Marketable debt securities	84,128	102,958
Bank line of credit	869,078	397,432
Term loans	477,890	524,807
Mortgage debt	722,153	963,786
Other debt	93,012	94,445
Warrants | Carrying Value
Summary of financial instruments
Derivative assets	20	55
Warrants | Level 3 | Fair Value
Summary of financial instruments
Derivative assets	20	55
Interest-rate swap contracts | Carrying Value
Summary of financial instruments
Derivative assets		196
Derivative liabilities	7,858	6,251
Interest-rate swap contracts | Level 2 | Fair Value
Summary of financial instruments
Derivative assets		196
Derivative liabilities	7,858	6,251
Currency Swap | Level 2 | Carrying Value
Summary of financial instruments
Derivative assets	3,017	1,551
Currency Swap | Level 2 | Fair Value
Summary of financial instruments
Derivative assets	$ 3,017	$ 1,551