Fair Value Measurements - Recurring (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cumulative gain (loss) adjustment for non performance risk	$ 5	$ 8
Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	43,863	49,182
Liabilities	106	118
Fair Value of debt securities transferred between level 1 and level 2			0
Cumulative gain (loss) adjustment for non performance risk		5	8
Fair Value, Measurements, Recurring | Derivative liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	87	98
Fair Value, Measurements, Recurring | Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	19	20
Fair Value, Measurements, Recurring | Retained Interest [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	17	17
Fair Value, Measurements, Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	2,010	1,704
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	264	308
Liabilities	0	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Derivative liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Retained Interest [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	47,150	44,176
Liabilities	5,464	4,318
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Derivative liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	5,445	4,298
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	19	20
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Retained Interest [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	10,155	9,061
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	4,633	12,098
Liabilities	12	15
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Derivative liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	12	15
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Retained Interest [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	17	17
Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	39	43
Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	(8,184)	(7,400)
Liabilities	(5,370)	(4,215)
Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring | Derivative liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	(5,370)	(4,215)
Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring | Other Liabilities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring | Retained Interest [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	(8,184)	(7,400)
US Corporate | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	24,285	23,787
US Corporate | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
US Corporate | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	21,073	20,659
US Corporate | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	3,212	3,128
US Corporate | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
State and municipal | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	5,584	5,749
State and municipal | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
State and municipal | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	5,032	5,171
State and municipal | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	552	578
State and municipal | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Residential Mortgage Backed Securities | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	1,225	1,725
Residential Mortgage Backed Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Residential Mortgage Backed Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	1,222	1,709
Residential Mortgage Backed Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	3	16
Residential Mortgage Backed Securities | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Commercial mortgage backed | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	2,597	3,063
Commercial mortgage backed | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Commercial mortgage backed | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	2,595	3,054
Commercial mortgage backed | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	2	9
Commercial mortgage backed | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Asset-backed | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	295	7,918
Asset-backed | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Asset-backed | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	183	343
Asset-backed | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	112	7,575
Asset-backed | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Corporate non US | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	1,050	1,135
Fair Value of debt securities transferred between level 1 and level 2		13
Corporate non US | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	13	0
Corporate non US | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	640	680
Corporate non US | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	397	455
Corporate non US | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Government - non-U.S. | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	1,468	1,796
Fair Value of debt securities transferred between level 1 and level 2		487
Government - non-U.S. | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	55	56
Government - non-U.S. | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	1,411	1,738
Government - non-U.S. | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	2	2
Government - non-U.S. | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
U.S. Government and federal agency | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	5,115	2,013
U.S. Government and federal agency | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
U.S. Government and federal agency | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	4,824	1,747
U.S. Government and federal agency | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	291	266
U.S. Government and federal agency | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Available-for-sale Securities | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	198	254
Available-for-sale Securities | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	177	231
Available-for-sale Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	15	14
Available-for-sale Securities | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	6	9
Available-for-sale Securities | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Trading [Member] | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	19	21
Trading [Member] | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	19	21
Trading [Member] | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Trading [Member] | Fair Value, Inputs, Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	0	0
Trading [Member] | Netting Adjustment Including Collateral | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets Fair Value Disclosure	$ 0	$ 0