Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value of Financial Instruments
Fixed rate debt	$ 4,900,000,000
Floating rate debt	1,600,000,000
Debt and preferred equity investments	1,357,203,000	985,942,000
Interest Rate Cap Expiring in June 2016 [Member]
Fair Value of Financial Instruments
Notional Value	30,000,000
Level 3
Fair Value of Financial Instruments
Fixed rate debt	5,300,000,000
Floating rate debt	1,600,000,000
Estimated fair value of debt and preferred equity investments, low end of range	1,300,000,000
Estimated fair value of debt and preferred equity investments, high end of range	1,400,000,000
Debt and preferred equity investments	$ 1,400,000,000