Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Fair Value of Financial Instruments
Marketable securities	$ 43,915	$ 45,138
Interest rate cap and swap agreements (included in other assets)	6	204
Debt and preferred equity investments	1,378,616	1,670,020
Secured Debt, Bearing Fixed Interest, Amount	5,990,813	5,816,010
Secured Debt, Bearing Variable Interest, Amount	1,130,887	1,205,494
Accrued Interest Payable And Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements	2,600	10,776
Liabilities Held for Sale
Fair Value of Financial Instruments
Interest rate cap and swap agreements	12,077
388-390 Greenwich Street
Fair Value of Financial Instruments
Secured Debt, Bearing Fixed Interest, Amount	1,004,000	1,004,000
Secured Debt, Bearing Variable Interest, Amount	446,000	446,000
388-390 Greenwich Street | Liabilities Held for Sale
Fair Value of Financial Instruments
Secured Debt, Bearing Fixed Interest, Amount	1,000,000
Secured Debt, Bearing Variable Interest, Amount	446,000
Carrying Value
Fair Value of Financial Instruments
Debt and preferred equity investments	1,378,616
Fixed rate debt	7,154,930	7,232,254
Variable rate debt	2,908,887	3,202,494
Total	10,063,817	10,434,748
Fair Value
Fair Value of Financial Instruments
Marketable securities	43,915	45,138
Total	10,412,799	10,770,574
Estimated fair value of debt and preferred equity investments, low end of range	1,400,000	1,700,000
Estimated fair value of debt and preferred equity investments, high end of range	1,500,000	1,800,000
Level 1
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in other assets)	0	0
Level 1 | Accrued Interest Payable And Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements	0	0
Level 1 | Liabilities Held for Sale
Fair Value of Financial Instruments
Interest rate cap and swap agreements	0
Level 2
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in other assets)	6	204
Level 2 | Accrued Interest Payable And Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements	2,600	10,776
Level 2 | Liabilities Held for Sale
Fair Value of Financial Instruments
Interest rate cap and swap agreements	12,077
Level 3
Fair Value of Financial Instruments
Marketable securities	0	0
Interest rate cap and swap agreements (included in other assets)	0	0
Level 3 | Accrued Interest Payable And Other Liabilities
Fair Value of Financial Instruments
Interest rate cap and swap agreements	0	0
Level 3 | Liabilities Held for Sale
Fair Value of Financial Instruments
Interest rate cap and swap agreements	0
Level 3 | Fair Value
Fair Value of Financial Instruments
Fixed rate debt	7,507,340	7,591,388
Variable rate debt	2,905,459	3,179,186
Equity marketable securities
Fair Value of Financial Instruments
Marketable securities	4,132	4,704
Equity marketable securities | Level 1
Fair Value of Financial Instruments
Marketable securities	4,132	4,704
Commercial mortgage-backed securities
Fair Value of Financial Instruments
Marketable securities	39,783	40,434
Commercial mortgage-backed securities | Level 2
Fair Value of Financial Instruments
Marketable securities	$ 39,783	$ 40,434