Level 4- Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	$ 35,924	$ 36,964
Fixed maturities, FVO	38	41
Marketable Securities	1,123	0
Available-for-sale Securities, Equity Securities	0	1,012
Derivative assets	12	10
Short-term investments	2,385	2,270
Total assets accounted for at fair value on a recurring basis	39,482	40,297
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(86)	(100)
Contingent consideration	(27)	(29)
Total liabilities accounted for at fair value on a recurring basis	(113)	(129)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	11	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	1	(3)
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)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(21)	(13)
Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(66)	(84)
Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	911	1,126
Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,144	1,260
Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,311	3,336
Corporate
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	12,634	12,804
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,082	1,110
Municipal
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	11,544	12,485
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,086	3,044
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	2,212	1,799
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	458	333
Fixed maturities, FVO	0	0
Available-for-sale Securities, Equity Securities	1,009	887
Short-term investments	874	1,098
Total assets accounted for at fair value on a recurring basis	2,341	2,318
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	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
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
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	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
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	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
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	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
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	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
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	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
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	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
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	458	333
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	33,547	34,679
Fixed maturities, FVO	38	41
Available-for-sale Securities, Equity Securities	49	49
Derivative assets	11	9
Short-term investments	1,511	1,172
Total assets accounted for at fair value on a recurring basis	35,156	35,950
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(88)	(101)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(88)	(101)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	11	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	1	(3)
Significant Observable Inputs (Level 2) | Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		(1)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(21)	(13)
Significant Observable Inputs (Level 2) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(68)	(85)
Significant Observable Inputs (Level 2) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	897	1,107
Significant Observable Inputs (Level 2) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,038	1,165
Significant Observable Inputs (Level 2) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	3,278	3,267
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	12,119	12,284
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,080	1,108
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	11,528	12,468
Significant Observable Inputs (Level 2) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,853	1,814
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,754	1,466
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,919	1,952
Fixed maturities, FVO	0	0
Available-for-sale Securities, Equity Securities	65	76
Derivative assets	1	1
Short-term investments	0	0
Total assets accounted for at fair value on a recurring basis	1,985	2,029
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	2	1
Contingent consideration	(27)	(29)
Total liabilities accounted for at fair value on a recurring basis	(25)	(28)
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	1
Significant Unobservable Inputs (Level 3) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		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
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	2	1
Significant Unobservable Inputs (Level 3) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	14	19
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	106	95
Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	33	69
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	515	520
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	2	2
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	16	17
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	1,233	1,230
Significant Unobservable Inputs (Level 3) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Fixed maturities, available-for-sale, at fair value (amortized cost of $54,478 and $53,805)	0	$ 0
Other investments | Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	$ 1