Credit Facilities (Outstanding Letters Of Credit And/Or Borrowings Of Group Credit Facility) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Commitment	$ 1,300,000,000	$ 1,300,000,000
In Use	127,000.000
Date of Expiry	9/30/2012
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Commitment	350,000,000	350,000,000
Wells Fargo Bank Group Credit Facility Tranche Two [Member]
Line of Credit Facility [Line Items]
Commitment	500,000,000	500,000,000
In Use	446,327,000	406,427,000
Date of Expiry	12/31/2012	12/31/2011
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment	850,000,000	850,000,000
In Use	$ 446,454,000	$ 406,427,000
Revolving Credit [Member]
Line of Credit Facility [Line Items]
Credit facility	1
Standby Letters Of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility	2