Borrowing Arrangements (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2013	$ 553,227,000	[1]
2014	339,668,000
2015	1,652,672,000
2016	442,518,000
2017	942,158,000
Thereafter	4,256,072,000	[1],[2]
Total maturities	8,186,315,000		6,180,443,000
Unrestricted cash and cash equivalents	67,908,000		45,807,000	21,812,000	107,397,000	107,397,000
Amount outstanding, net of cash and cash equivalents	472,800,000
Interest rate (as a percent)	9.00%			7.125%
Minimum percentage of total unencumbered assets as a percentage of unsecured debt required to be maintained (as a percent)	150.00%
Line of Credit
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	540,727,000		455,578,000
Mortgage loans
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	23,200,000
6.90% Senior Notes due 2037
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	52,400,000
Interest rate (as a percent)	6.90%
6.59% senior notes due 2038
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Thereafter	23,000,000
Interest rate (as a percent)	6.59%
Principal Amount Due at Maturity
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2013	501,029,000	[1]
2014	291,708,000
2015	1,073,272,000
2016	410,917,000
2017	922,731,000
Thereafter	4,098,227,000	[1],[2]
Total maturities	7,297,884,000
Unsecured Revolving Credit Facility
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2013	0	[1],[3]
2014	0	[3]
2015	540,727,000	[3]
2016	0	[3]
2017	0	[3]
Thereafter	0	[1],[2],[3]
Total maturities	540,727,000	[3]
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2013	52,198,000	[1]
2014	47,960,000
2015	38,673,000
2016	31,601,000
2017	19,427,000
Thereafter	157,845,000	[1],[2]
Total maturities	347,704,000
Line of Credit
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Unrestricted cash and cash equivalents	$ 67,900,000

[1] Excludes debt related to one property classified as held for sale as of December 31, 2012. The total mortgage debt for this property as of December 31, 2012 was $23.2 million and is scheduled to mature in 2013.

[2] Includes $52.4 million aggregate principal amount of our 6.90% senior notes due 2037 that are subject to repurchase, at the option of the holders, on October 1 in each of the years 2017 and 2027, and $23.0 million aggregate principal amount of our 6.59% senior notes due 2038 that are subject to repurchase, at the option of the holders, on July 7 in each of the years 2013, 2018, 2023 and 2028.

[3] At December 31, 2012, we had $67.9 million of unrestricted cash and cash equivalents, for $472.8 million of net borrowings outstanding under our unsecured revolving credit facility.