Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments	$ 48,439	$ 47,359
Domestic Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Domestic Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	20,580	20,535
Domestic Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	3,552	3,235
Domestic Corporate Debt Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Domestic Corporate Debt Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	24,132	23,770
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	3,088	2,824
Commercial Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	6	4
Commercial Mortgage Backed Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Commercial Mortgage Backed Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	3,094	2,828
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	4,469	3,157
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	77	77
US States and Political Subdivisions Debt Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
US States and Political Subdivisions Debt Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	4,546	3,234
Foreign Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	71	71
Foreign Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	1,132	1,058
Foreign Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	1,212	1,204
Foreign Corporate Debt Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Foreign Corporate Debt Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	2,415	2,333
Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	715	930
Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	5,023	4,040
Asset-backed Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Asset-backed Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	5,738	4,970
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	702	1,003
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	1,019	1,444
Foreign Government Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	42	84
Foreign Government Debt Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Foreign Government Debt Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	1,763	2,531
US Treasury and Government [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
US Treasury and Government [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	3,288	3,805
US Treasury and Government [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	277	253
US Treasury and Government [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
US Treasury and Government [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	3,565	4,058
Retained Interest [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Retained Interest [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Retained Interest [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	83	35
Retained Interest [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Retained Interest [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	83	35
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	569	715
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	14	18
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	13	17
Available-for-sale Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Available-for-sale Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	596	750
Trading [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	245	241
Trading [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Trading [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Trading [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Trading [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	245	241
Derivatives [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivatives Liabilities	0	0
Derivative Assets	0	0
Derivatives [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivatives Liabilities	3,040	4,503
Derivative Assets	10,934	14,830
Derivatives [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivatives Liabilities	20	20
Derivative Assets	280	160
Derivatives [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivatives Liabilities	(2,908)	(4,025)
Derivative Assets	(7,657)	(5,319)
Derivatives [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivatives Liabilities	152	498
Derivative Assets	3,557	9,671
Other Items [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	0	0
Other Liabilities	0	0
Other Items [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	0	0
Other Liabilities	23	25
Other Items [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	432	388
Other Liabilities	0	0
Other Items [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	0	0
Other Liabilities	0	0
Other Items [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	432	388
Other Liabilities	23	25
Total [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	1,587	2,030
Total Liabilities	0	0
Total [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	47,401	51,169
Total Liabilities	3,063	4,528
Total [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	11,097	9,538
Total Liabilities	20	20
Total [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	(7,657)	(5,319)
Total Liabilities	(2,908)	(4,025)
Total [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	52,428	57,418
Total Liabilities	175	523
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	2,162	2,568
Residential Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	100	41
Residential Mortgage Backed Securities [Member] | Netting Adjustment [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0
Residential Mortgage Backed Securities [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	$ 2,262	$ 2,609