Fair Value Measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value of Financial Instruments
Marketable securities	$ 46,432	$ 39,429
Interest rate cap and swap agreements (included in other assets)	7	2,174
Interest rate cap swap agreements (included in accrued interest payable and other liabilities)	18,019	14,728
Debt and preferred equity investments	1,501,619	1,408,804
Carrying Value
Fair Value of Financial Instruments
Fixed rate debt	7,137,995	6,140,786
Variable rate debt	2,992,508	2,291,943
Total	10,130,503	8,432,729
Fair Value
Fair Value of Financial Instruments
Marketable securities	46,432	39,429
Total	10,626,588	8,881,188
Estimated fair value of debt and preferred equity investments, low end of range	1,700,000	1,500,000
Estimated fair value of debt and preferred equity investments, high end of range	1,900,000	1,800,000
Level 1
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in other assets)	0	0
Interest rate cap swap agreements (included in accrued interest payable and other liabilities)	0	0
Level 2
Fair Value of Financial Instruments
Interest rate cap and swap agreements (included in other assets)	7	2,174
Interest rate cap swap agreements (included in accrued interest payable and other liabilities)	18,019	14,728
Level 3
Fair Value of Financial Instruments
Marketable securities	0	0
Interest rate cap and swap agreements (included in other assets)	0	0
Interest rate cap swap agreements (included in accrued interest payable and other liabilities)	0	0
Level 3 | Carrying Value
Fair Value of Financial Instruments
Debt and preferred equity investments	1,501,619	1,408,804
Level 3 | Fair Value
Fair Value of Financial Instruments
Fixed rate debt	7,610,830	6,565,236
Variable rate debt	3,015,758	2,315,952
Equity marketable securities
Fair Value of Financial Instruments
Marketable securities	4,115	4,332
Equity marketable securities | Level 1
Fair Value of Financial Instruments
Marketable securities	4,115	4,332
Mortgage-backed securities
Fair Value of Financial Instruments
Marketable securities	42,317	35,097
Mortgage-backed securities | Level 2
Fair Value of Financial Instruments
Marketable securities	$ 42,317	$ 35,097