Fair Value of Financial Instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Fair Value of Financial Instruments
Fixed rate debt	$ 5,800,000,000
Amount swapped	30,000,000
Floating rate debt	900,000,000
Debt and preferred equity investments	1,443,834,000	1,357,203,000
Level 3
Fair Value of Financial Instruments
Fixed rate debt	6,300,000,000
Floating rate debt	900,000,000
Debt and preferred equity investments	1,400,000,000
Level 3 | Minimum
Fair Value of Financial Instruments
Debt and preferred equity investments	1,400,000,000
Level 3 | Maximum
Fair Value of Financial Instruments
Debt and preferred equity investments	$ 1,500,000,000