Fair Value of Financial Instruments (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Fair Value of Financial Instruments
Fixed rate debt	$ 5,800,000,000
Floating rate debt	900,000,000
Debt and preferred equity investments	1,443,834,000	1,357,203,000
Interest Rate Cap expiring in June, 2016
Fair Value of Financial Instruments
Notional Value	30,000,000
Level 3
Fair Value of Financial Instruments
Fixed rate debt	6,300,000,000
Floating rate debt	900,000,000
Estimated fair value of debt and preferred equity investments, low end of range	1,400,000,000
Estimated fair value of debt and preferred equity investments, high end of range	1,500,000,000
Debt and preferred equity investments	$ 1,400,000,000