Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets accounted for at fair value on a recurring basis
Total fixed maturities	$ 36,964	$ 32,182
Fixed maturities, FVO	41	211
Equity securities, AFS	1,012	945
Derivative assets	10	(415)
Short-term investments	2,270	1,895
Total assets accounted for at fair value on a recurring basis	40,297	34,818
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(100)	(112)
Contingent consideration	(29)	(25)
Total liabilities accounted for at fair value on a recurring basis	(129)	(137)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	9	18
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(3)	(14)
Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	(1)	23
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(13)	10
Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	(457)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(84)	(108)
Other derivative contracts
Assets accounted for at fair value on a recurring basis
Derivative assets		1
Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,126	1,389
Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,260	976
Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	3,336	2,790
Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	12,804	10,973
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,110	826
Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	12,485	10,297
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	3,044	3,006
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,799	1,925
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	333	390
Fixed maturities, FVO	0	1
Equity securities, AFS	887	801
Short-term investments	1,098	241
Total assets accounted for at fair value on a recurring basis	2,318	1,433
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other derivative contracts
Assets accounted for at fair value on a recurring basis
Derivative assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	333	390
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	34,679	29,666
Fixed maturities, FVO	41	199
Equity securities, AFS	49	89
Derivative assets	9	(416)
Short-term investments	1,172	1,654
Total assets accounted for at fair value on a recurring basis	35,950	31,192
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(101)	(121)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(101)	(121)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	9	18
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(3)	(14)
Significant Observable Inputs (Level 2) | Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	(1)	23
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(13)	10
Significant Observable Inputs (Level 2) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	(457)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(85)	(117)
Significant Observable Inputs (Level 2) | Other derivative contracts
Assets accounted for at fair value on a recurring basis
Derivative assets		0
Significant Observable Inputs (Level 2) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,107	1,344
Significant Observable Inputs (Level 2) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,165	822
Significant Observable Inputs (Level 2) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	3,267	2,731
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	12,284	10,459
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,108	779
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	12,468	10,251
Significant Observable Inputs (Level 2) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,814	1,745
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,466	1,535
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,952	2,126
Fixed maturities, FVO	0	11
Equity securities, AFS	76	55
Derivative assets	1	1
Short-term investments	0	0
Total assets accounted for at fair value on a recurring basis	2,029	2,193
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	1	9
Contingent consideration	(29)	(25)
Total liabilities accounted for at fair value on a recurring basis	(28)	(16)
Significant Unobservable Inputs (Level 3) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1
Significant Unobservable Inputs (Level 3) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Significant Unobservable Inputs (Level 3) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	1	9
Significant Unobservable Inputs (Level 3) | Other derivative contracts
Assets accounted for at fair value on a recurring basis
Derivative assets		1
Significant Unobservable Inputs (Level 3) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	19	45
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	95	154
Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	69	59
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	520	514
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	2	47
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	17	46
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,230	1,261
Significant Unobservable Inputs (Level 3) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	$ 0	$ 0