Credit Agreements (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2011
Credit Agreements Details [Line Items]
Total amount of credit facilities available	$ 850
Outstanding Unsecured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	137
Outstanding Secured Letter Of Credit [Member]
Credit Agreements Details [Line Items]
Letters Of Credit Outstanding Amount	410
Combined Credit Facility [Member]
Credit Agreements Details [Line Items]
Maturity date of facility	Nov. 14, 2014
Modification date	11/14/2012
Amount of unsecured borrowing capacity	100
Total amount of credit facilities available	300	250
Committed Secured Letter Of Credit Facility A [Member]
Credit Agreements Details [Line Items]
Maturity date of facility	Dec. 04, 2016
Total amount of credit facilities available	250
Committed Secured Letter Of Credit Facility B [Member]
Credit Agreements Details [Line Items]
Maturity date of facility	Dec. 31, 2014
Total amount of credit facilities available	$ 200