Schedule II - Condensed Financial Information (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Short-term and Long-term Debt Outstanding	$ 5,200,000,000
Credit Facility Remaining Borrowing Capacity	5,300,000,000
Revolving Credit And Letter Of Credit Facility [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Initiation Date	2012-12
Line of Credit Facility, Expiration Date	2017-12
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	This agreement has certain covenants, including a financial covenant requiring the Company to maintain a total debt-to-adjusted capital ratio at or below 0.50 to 1.00
Revolving Credit And Letter Of Credit Facility [Member] | Minimum [Member]
Condensed Financial Statements, Captions [Line Items]
Maximum debt-to-adjusted capital ratio convenant	50.00%
Revolving Credit And Letter Of Credit Facility [Member] | Maximum [Member]
Condensed Financial Statements, Captions [Line Items]
Maximum debt-to-adjusted capital ratio convenant	100.00%
Letter of Credit [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000
Letters of Credit Outstanding, Amount	66,000,000
Primary Commitment Holders [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Individual Bank Percentage Commitment	12.00%
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Number of Participating Banks	3
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Number of Participating Banks	13
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Term of Agreement	5 years
Line of Credit Facility, Number of Participating Banks	16
Line of Credit Facility, Optional Maximum Borrowing Capacity	2,000,000,000
Short-term and Long-term Debt Outstanding	5,200,000,000
Letters of Credit Outstanding, Amount	66,000,000
Credit Facility Remaining Borrowing Capacity	$ 5,300,000,000
Parent Company [Member] | Revolving Credit And Letter Of Credit Facility [Member] | Minimum [Member]
Condensed Financial Statements, Captions [Line Items]
Maximum debt-to-adjusted capital ratio convenant	50.00%
Parent Company [Member] | Revolving Credit And Letter Of Credit Facility [Member] | Maximum [Member]
Condensed Financial Statements, Captions [Line Items]
Maximum debt-to-adjusted capital ratio convenant	100.00%
Parent Company [Member] | Primary Commitment Holders [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Number of Participating Banks	3
Line of Credit Facility, Individual Bank Percentage Commitment	12.00%
Parent Company [Member] | Minor Commitment Holders [Member]
Condensed Financial Statements, Captions [Line Items]
Line of Credit Facility, Number of Participating Banks	13