Unsecured Revolving Credit Facility (Narrative) (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Borrowing capacity under the credit facility	$ 600,000,000
Expandable borrowing capacity under the credit facility	1,000,000,000
Credit facility extension fee, basis points	20
Number of lending institutions	20
Credit facility mature date	October 2015
Line of credit facility, bid feature, current borrowing capacity	300,000,000
Terms of the unsecured facility	The terms of the unsecured facility include certain restrictions and covenants which limit, among other things the incurrence of additional indebtedness, the incurrence of liens and the disposition of real estate properties (to the extent that: (i) such property dispositions cause the Company to default on any of the financial ratios of the facility described below, or (ii) the property dispositions are completed while the Company is under an event of default under the facility, unless, under certain circumstances, such disposition is being carried out to cure such default), and which require compliance with financial ratios relating to the maximum leverage ratio, the maximum amount of secured indebtedness, the minimum amount of tangible net worth, the minimum amount of fixed charge coverage, the maximum amount of unsecured indebtedness, the minimum amount of unencumbered property interest coverage and certain investment limitations.
Terms of dividend restriction	If an event of default has occurred and is continuing, the Company will not make any excess distributions except to enable the Company to continue to qualify as a REIT under the Code.
Outstanding borrowings under the facility	27,000,000	228,000,000
Previous Credit Facility [Member]
Borrowing capacity under the credit facility	775,000,000
Interest rate on outstanding borrowings	LIBOR plus 55
Money Market Loan [Member]
Borrowing capacity under the credit facility	75,000,000
Spread over LIBOR	55
Outstanding borrowings under the facility	$ 0	$ 0
Spread over LIBOR	55
Maturity period of the unsecured borrowing	30 days or less