Debt - Revolving Credit and Letter of Credit (Details) (USD $)	9 Months Ended
	Sep. 30, 2013
Line of Credit Facility [Line Items]
Short-term and Long-term Debt Outstanding	$ 5,200,000,000
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Initiation Date	Dec. 01, 2012
Line of Credit Facility, Expiration Date	Dec. 01, 2017
Line of Credit Facility, Term of Agreement	5 years
Line of Credit Facility, Maximum Borrowing Capacity	1,500,000,000
Line of Credit Facility, Number of Participating Banks	16
Line of Credit Facility, Optional Maximum Borrowing Capacity	2,000,000,000
Line of Credit Facility, Covenant Terms	maintain a total debt to adjusted capital ratio at or below 0.50 to 1.00
Credit Facility Remaining Borrowing Capacity	5,700,000,000
Revolving Credit And Letter Of Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Maximum debt-to-adjusted capital ratio convenant	50.00%
Revolving Credit And Letter Of Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Maximum debt-to-adjusted capital ratio convenant	100.00%
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000
Letters of Credit Outstanding, Amount	$ 39,000,000
Primary Commitment Holders [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	3
Line of Credit Facility, Individual Bank Percentage Commitment	12.00%
Minor Commitment Holders [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	13
Revolving Credit And Letter Of Credit Agreement Remaining Thirteen Banks Percentage Committment	64.00%