Recent Financing Transactions Recent Financing Transactions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Letter of Credit Facility [Member]
Line of Credit Facility [Line Items]
Letters of Credit Outstanding, Amount	$ 289
Unsecured credit facility, $630 million, expiring July 2013 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of credit facility, borrowing capacity replaced	630
Line of Credit Facility, Expiration Date	Jul. 31, 2013
Unsecured credit facility, $600 million, expiring March 2018 [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	600
Line of Credit Facility, Expiration Date	Mar. 31, 2018
Line of Credit Facility, Amount Outstanding	0
Debt to capitalization ratio	0.65
Unsecured credit facility, $600 million, expiring March 2018 [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Amounts Supported	$ 312