Fair Value Measurements (Changes in Level 3 Instruments) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Cash Accruals Not Included In Schedule Assets Measured For Fair Value On Recurring Basis	$ 12	$ 9	$ 12	$ 9
Level 3 | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	222
Net realized/unrealized gains (losses) included in earnings	10
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0
Purchases	0
Sales	0
Settlements	0
Transfers into Level 3	0
Transfers out of Level 3	0
Changes in Level 3, ending balance	233		233
Level 3 | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	4,083	4,395	4,246	4,128
Net realized/unrealized gains (losses) included in earnings	29	11	(16)	63
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	10	7	(65)	206
Purchases	74	103	262	475
Sales	(29)	(57)	(194)	(300)
Settlements	(45)	(124)	(127)	(211)
Transfers into Level 3	41	41	83	181
Transfers out of Level 3	(70)	(64)	(96)	(230)
Changes in Level 3, ending balance	4,094	4,312	4,094	4,312
Net change in unrealized gains (losses) relating to instruments still held	14	(1)	(26)	17
Level 3 | Recurring | Derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	72	29	36	20
Net realized/unrealized gains (losses) included in earnings	16	1	15	8
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	1	1	3	1
Purchases	(1)	(2)	0	(1)
Sales	0	0	0	0
Settlements	(1)	0	(9)	3
Transfers into Level 3	0	0	42	(1)
Transfers out of Level 3	0	0	0	(1)
Changes in Level 3, ending balance	86	29	86	29
Net change in unrealized gains (losses) relating to instruments still held	14	(1)	11	13
Level 3 | Recurring | Retained interest
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance		1	0	1
Net realized/unrealized gains (losses) included in earnings		0		0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income		0		0
Purchases		0		0
Sales		0		0
Settlements		(1)		(1)
Transfers into Level 3		0		0
Transfers out of Level 3		0		0
Changes in Level 3, ending balance		0		0
Net change in unrealized gains (losses) relating to instruments still held		0		0
Level 3 | Recurring | Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	222	217	277	201
Net realized/unrealized gains (losses) included in earnings	10	5	(24)	16
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0	0	0
Purchases	0	0	0	20
Sales	0	0	(21)	(15)
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Changes in Level 3, ending balance	233	222	233	222
Net change in unrealized gains (losses) relating to instruments still held	0	0	(37)	4
Level 3 | US Corporate | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	3,024	3,060	3,056	2,787
Net realized/unrealized gains (losses) included in earnings	5	4	2	22
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	(7)	(4)	(93)	115
Purchases	74	102	255	441
Sales	(29)	(57)	(84)	(213)
Settlements	(37)	(90)	(93)	(158)
Transfers into Level 3	35	32	35	170
Transfers out of Level 3	(15)	(16)	(28)	(133)
Changes in Level 3, ending balance	3,050	3,031	3,050	3,031
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | State and municipal | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	101	110	115	96
Net realized/unrealized gains (losses) included in earnings	0	0	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	1	2	(3)	9
Purchases	0	2	0	12
Sales	0	0	0	0
Settlements	(5)	(1)	(15)	(4)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(17)	0	(17)	0
Changes in Level 3, ending balance	81	113	81	113
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | Residential Mortgage Backed Securities | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	2	66	16	86
Net realized/unrealized gains (losses) included in earnings	0	0	5	1
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0	(4)	0
Purchases	0	0	0	0
Sales	0	0	(15)	(16)
Settlements	(1)	(3)	(1)	(8)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(47)	0	(47)
Changes in Level 3, ending balance	2	16	2	16
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | Commercial mortgage backed | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	2	12	9	10
Net realized/unrealized gains (losses) included in earnings	0	0	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	(7)	0
Settlements	0	(2)	0	(2)
Transfers into Level 3	0	0	0	2
Transfers out of Level 3	0	0	0	0
Changes in Level 3, ending balance	1	10	1	10
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | Asset-backed | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	76	130	123	145
Net realized/unrealized gains (losses) included in earnings	(2)	1	(16)	3
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	2	(5)	6
Purchases	0	0	0	0
Sales	0	0	(12)	0
Settlements	0	(4)	(3)	(15)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(36)	0	(49)	(10)
Changes in Level 3, ending balance	38	129	38	129
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | Corporate - non-U.S. | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	283	511	337	515
Net realized/unrealized gains (losses) included in earnings	0	0	0	13
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0	(4)	43
Purchases	1	1	1	1
Sales	0	0	(50)	(54)
Settlements	(1)	(23)	(1)	(24)
Transfers into Level 3	0	0	0	1
Transfers out of Level 3	0	0	0	(6)
Changes in Level 3, ending balance	283	489	283	489
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | Government - non-U.S. | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	2	1	2	31
Net realized/unrealized gains (losses) included in earnings	0	0	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(2)	(1)	(2)	(31)
Changes in Level 3, ending balance	0	0	0	0
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | U.S. Government and federal agency | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	293	249	266	225
Net realized/unrealized gains (losses) included in earnings	0	0	0	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	16	6	44	32
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	(1)	0
Transfers into Level 3	0	9	0	9
Transfers out of Level 3	0	0	0	(2)
Changes in Level 3, ending balance	309	264	309	264
Net change in unrealized gains (losses) relating to instruments still held	0	0	0	0
Level 3 | Available-for-sale Securities | Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Changes in Level 3, beginning balance	6	9	9	11
Net realized/unrealized gains (losses) included in earnings	0	0	2	0
Net realized/unrealized gains (losses) included in accumulated other comprehensive income	(1)	0	(3)	0
Purchases	0	0	6	2
Sales	0	0	(5)	(2)
Settlements	0	0	(4)	(2)
Transfers into Level 3	6	0	6	0
Transfers out of Level 3	0	0	0	0
Changes in Level 3, ending balance	10	9	10	9
Net change in unrealized gains (losses) relating to instruments still held	$ 0	$ 0	$ 0	$ 0