Unsecured Lines Of Credit Payable (Lines Of Credit Unused And Available) (Details) (USD $)	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Line of Credit Facility [Line Items]
Committed capacity	$ 475,000,000	$ 337,000,000	$ 337,000,000
Borrowings outstanding	(99,000,000)	(100,000,000)	(128,000,000)
Credit Facility No. 1 [Member]
Line of Credit Facility [Line Items]
Committed capacity	75,000,000
Borrowings outstanding	74,000,000	0
Letters of credit issued	800,000
Unused and available	200,000
Credit Facility No. 2 [Member]
Line of Credit Facility [Line Items]
Committed capacity	400,000,000
Borrowings outstanding	25,000,000	(100,000,000)
Letters of credit issued	0
Unused and available	$ 375,000,000