Credit Facilities (Summary Of Outstanding Letters Of Credit And/Or Borrowings Of Group Credit Facility) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Line of Credit Facility [Line Items]
Commitment	$ 1,250,000
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	501,853	502,059
Date of Expiry	Dec. 31, 2014	Dec. 31, 2014
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment	800,000	800,000
In Use	$ 501,853	$ 502,059