Fair Value of Financial Instruments (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010
Fair Value of Financial Instruments
Fixed rate debt	$ 4,800,000,000
Floating rate debt	1,300,000,000
Estimated fair value of debt and preferred equity investments, low end of range	838,100,000
Estimated fair value of debt and preferred equity investments, high end of range	936,600,000
Debt and preferred equity investments	985,942,000	963,772,000
Level 3
Fair Value of Financial Instruments
Fixed rate debt	5,200,000,000
Floating rate debt	$ 1,200,000,000