Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities	$ 45,733	$ 43,981
Fair Value Of Securities Transferred Between Level One And Level Two	0
Cumulative Gain (Loss) Adjustment For Non Performance Risk	30	(7)
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	19,590	18,788
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	3,104	2,953
Domestic Corporate Debt Securities [Member] | Netting Adjustment Including Collateral [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	22,694	21,741
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	3,112	3,025
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	11	10
Commercial Mortgage Backed Securities [Member] | Netting Adjustment Including Collateral [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	3,123	3,035
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	4,834	4,193
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	560	96
US States and Political Subdivisions Debt Securities [Member] | Netting Adjustment Including Collateral [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	5,394	4,289
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	52	61
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	678	645
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	1,072	1,064
Foreign Corporate Debt Securities [Member] | Netting Adjustment Including Collateral [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	1,802	1,770
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	430	489
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	6,908	6,898
Asset-backed Securities [Member] | Netting Adjustment Including Collateral [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	7,338	7,387
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	1,334	1,590
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	823	789
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	1	31
Foreign Government Debt Securities [Member] | Netting Adjustment Including Collateral [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	2,158	2,410
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	505	545
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	232	225
US Treasury and Government [Member] | Netting Adjustment Including Collateral [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	737	770
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	75	72
Retained Interest [Member] | Netting Adjustment Including Collateral [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	75	72
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	434	475
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	25	31
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	11	11
Available-for-sale Securities [Member] | Netting Adjustment Including Collateral [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	470	517
Trading [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	66	78
Trading [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investments, Fair Value Disclosure	2	2
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 Including Collateral [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	68	80
Derivatives [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Derivatives [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets	7,683	8,304
Derivative liabilities	4,692	5,409
Derivatives [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets	163	175
Derivative liabilities	15	20
Derivatives [Member] | Netting Adjustment Including Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets	(6,300)	(6,739)
Derivative liabilities	(3,895)	(4,355)
Derivatives [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets	1,546	1,740
Derivative liabilities	812	1,074
Other 1 [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 1 [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	0	0
Other Liabilities	1,183	1,170
Other 1 [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	288	494
Other Liabilities	0	0
Other 1 [Member] | Netting Adjustment Including Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	0	0
Other Liabilities	0	0
Other 1 [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Other Assets	288	494
Other Liabilities	1,183	1,170
Total [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	1,886	2,204
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	39,475	38,635
Total Liabilities	5,875	6,579
Total [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	12,506	12,115
Total Liabilities	15	20
Total [Member] | Netting Adjustment Including Collateral [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	(6,300)	(6,739)
Total Liabilities	(3,895)	(4,355)
Total [Member] | Net Balance [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total Assets	47,567	46,215
Total Liabilities	1,995	2,244
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	1,793	1,824
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	81	86
Residential Mortgage Backed Securities [Member] | Netting Adjustment Including Collateral [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	$ 1,874	$ 1,910