Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Summary of financial instruments
Bank line of credit	$ 1,372,032	$ 397,432
Senior unsecured notes	8,229,731	9,120,107
Mortgage debt	762,715	932,212
Other debt	93,876	94,445
Carrying Value
Summary of financial instruments
Loans receivable, net	682,994	768,743
Marketable debt securities	82,081	102,958
Marketable equity securities	37	39
Bank line of credit	1,372,032	397,432
Term loans	462,181	524,807
Senior unsecured notes	8,229,731	9,120,107
Mortgage debt	762,715	932,212
Other debt	93,876	94,445
Warrants | Carrying Value
Summary of financial instruments
Derivative assets	18	55
Interest-rate swap contracts | Carrying Value
Summary of financial instruments
Derivative assets		196
Derivative liabilities	6,558	6,251
Level 1 | Fair Value
Summary of financial instruments
Marketable equity securities	37	39
Senior unsecured notes	8,737,688	9,390,668
Level 2 | Fair Value
Summary of financial instruments
Loans receivable, net	688,804	770,052
Marketable debt securities	82,081	102,958
Bank line of credit	1,372,032	397,432
Term loans	462,181	524,807
Mortgage debt	813,236	963,786
Other debt	93,876	94,445
Level 2 | Interest-rate swap contracts | Fair Value
Summary of financial instruments
Derivative assets		196
Derivative liabilities	6,558	6,251
Level 2 | Currency Swap | Carrying Value
Summary of financial instruments
Derivative assets	2,901	1,551
Level 2 | Currency Swap | Fair Value
Summary of financial instruments
Derivative assets	2,901	1,551
Level 3 | Warrants | Fair Value
Summary of financial instruments
Derivative assets	$ 18	$ 55