Debt - Revolving Credit and Letter of Credit (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Dec. 31, 2012
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
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	5,900,000,000
Short-term and Long-term Debt Outstanding	5,200,000,000
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	$ 23,000,000
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	3
Line of Credit Facility, Individual Bank Percentage Committment	12.00%
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Number of Participating Banks	13