Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings Of Group Credit Facility) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jul. 27, 2007
Line of Credit Facility [Line Items]
Commitment	$ 1,100,000
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Commitment	200,000	200,000
Wells Fargo Bank Group Credit Facility Tranche Two Issuance One [Member]
Line of Credit Facility [Line Items]
Commitment	600,000	600,000
In Use	444,012	502,059
Date of Expiry	Dec. 31, 2015	Dec. 31, 2014
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment	800,000	800,000	850,000
In Use	$ 444,012	$ 502,059