Borrowing Arrangements (Scheduled Maturities) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2014	$ 141,609,000
2015	970,671,000
2016	994,625,000
2017	562,036,000	[1]
2018	1,474,285,000
Thereafter	5,177,251,000	[2]
Total maturities	9,320,477,000		8,186,315,000
Unrestricted cash and cash equivalents	94,816,000		67,908,000	45,807,000	21,812,000
Amount outstanding, net of cash and cash equivalents	281,500,000
Minimum percentage of total unencumbered assets as a percentage of unsecured debt required to be maintained (as a percent)	150.00%
Unsecured revolving credit facilities
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	376,343,000		540,727,000
Mortgage loans held for sale
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
Total maturities	13,100,000		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
2014	95,657,000
2015	929,941,000
2016	960,917,000
2017	540,072,000	[1]
2018	1,082,496,000
Thereafter	5,030,288,000	[2]
Total maturities	8,639,371,000
Scheduled Periodic Amortization
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2014	45,952,000
2015	40,730,000
2016	33,708,000
2017	21,964,000	[1]
2018	15,446,000
Thereafter	146,963,000	[2]
Total maturities	304,763,000
Unsecured revolving credit facilities
Scheduled maturities of borrowing arrangements and other provisions excluding capital lease obligations
2014	0	[3]
2015	0	[3]
2016	0	[3]
2017	0	[3]
2018	376,343,000	[3]
Thereafter	0	[3]
Total maturities	$ 376,343,000	[3]

[1] Excludes $13.1 million of mortgage debt related to a real estate asset classified as held for sale as of December 31, 2013 that is scheduled to mature in 2017.

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

[3] At December 31, 2013, we had $94.8 million of unrestricted cash and cash equivalents, for $281.5 million of net borrowings outstanding under our unsecured revolving credit facility.