Fair Value of Financial Instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value of Financial Instruments
Fixed rate debt	$ 4,900,000,000
Amount swapped	30,000,000
Floating rate debt	1,600,000,000
Debt and preferred equity investments	1,357,203,000	985,942,000
Level 3
Fair Value of Financial Instruments
Fixed rate debt	5,300,000,000
Floating rate debt	1,600,000,000
Debt and preferred equity investments	1,400,000,000
Level 3 | Minimum
Fair Value of Financial Instruments
Debt and preferred equity investments	1,300,000,000
Level 3 | Maximum
Fair Value of Financial Instruments
Debt and preferred equity investments	$ 1,400,000,000