Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Cumulative Gain (Loss) Adjustment For Non Performance Risk	$ (9)
Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	46,823	$ 33,512
Liabilities	1,806	1,895
Recurring | Derivative liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	699	713
Recurring | Other Liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	1,107	1,182
Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	454	1,281
Recurring | Other Assets
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets		259
US Corporate | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	25,099	22,358
State and municipal | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	4,556	4,245
Mortgage and asset-backed | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	2,966	3,116
Corporate - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	11,873	847
Fair Value Of Securities Transferred Between Level 1 And Level 2	12
Government - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	752	415
U.S. Government and federal agency | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	797	776
Fair Value Of Securities Transferred Between Level 1 And Level 2	50
Equity | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	325	217
Level 1 | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	301	260
Liabilities	0	0
Level 1 | Recurring | Derivative liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Level 1 | Recurring | Other Liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Level 1 | Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Level 1 | Recurring | Other Assets
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets		0
Level 1 | US Corporate | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Level 1 | State and municipal | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Level 1 | Mortgage and asset-backed | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Level 1 | Corporate - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	12
Level 1 | Government - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	5
Level 1 | U.S. Government and federal agency | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	49
Level 1 | Equity | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	301	194
Level 2 | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	48,902	35,331
Liabilities	5,094	6,860
Level 2 | Recurring | Derivative liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	3,987	5,677
Level 2 | Recurring | Other Liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	1,107	1,182
Level 2 | Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	6,947	7,312
Level 2 | Recurring | Other Assets
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets		0
Level 2 | US Corporate | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	21,600	19,351
Level 2 | State and municipal | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	4,502	4,215
Level 2 | Mortgage and asset-backed | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	2,958	3,084
Level 2 | Corporate - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	11,636	544
Level 2 | Government - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	752	410
Level 2 | U.S. Government and federal agency | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	492	404
Level 2 | Equity | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	14	9
Level 3 | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	4,141	4,033
Liabilities	1	4
Level 3 | Recurring | Derivative liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	1	4
Level 3 | Recurring | Other Liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Level 3 | Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	27	79
Level 3 | Recurring | Other Assets
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets		259
Level 3 | US Corporate | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	3,499	3,006
Level 3 | State and municipal | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	55	30
Level 3 | Mortgage and asset-backed | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	8	32
Level 3 | Corporate - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	237	290
Level 3 | Government - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Level 3 | U.S. Government and federal agency | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	305	323
Level 3 | Equity | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	10	13
Netting Adjustment | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	(6,521)	(6,110)
Liabilities	(3,289)	(4,968)
Netting Adjustment | Recurring | Derivative liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	(3,289)	(4,968)
Netting Adjustment | Recurring | Other Liabilities
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Liabilities	0	0
Netting Adjustment | Recurring | Derivatives
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	(6,521)	(6,110)
Netting Adjustment | Recurring | Other Assets
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets		0
Netting Adjustment | US Corporate | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Netting Adjustment | State and municipal | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Netting Adjustment | Mortgage and asset-backed | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Netting Adjustment | Corporate - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Netting Adjustment | Government - non-U.S. | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Netting Adjustment | U.S. Government and federal agency | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	0	0
Netting Adjustment | Equity | Recurring
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Assets	$ 0	$ 0