Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018
Assets accounted for at fair value on a recurring basis
Total fixed maturities	$ 42,148	$ 35,652
Fixed maturities, FVO	11	22
Equity securities, at fair value	1,657	1,214	$ 1,000
Derivative assets	12	7
Short-term investments	2,921	4,283
Total assets accounted for at fair value on a recurring basis	46,749	41,178
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(78)	(68)
Contingent consideration	(22)	(35)
Total liabilities accounted for at fair value on a recurring basis	(100)	(103)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	11	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(1)	(2)
Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		3
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(15)	1
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		(2)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(2)	(5)
Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(60)	(62)
Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,476	1,276
Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	2,183	1,437
Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	4,338	3,552
Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	17,396	13,398
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,123	847
Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	9,498	10,346
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	4,869	3,279
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,265	1,517
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	330	330
Fixed maturities, FVO	0	0
Equity securities, at fair value	1,401	1,093
Derivative assets		0
Short-term investments	1,028	1,039
Total assets accounted for at fair value on a recurring basis	2,759	2,462
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
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	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	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
Total fixed maturities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Collateralized loan obligations (CLOs)
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	330	330
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	40,404	33,757
Fixed maturities, FVO	11	22
Equity securities, at fair value	183	44
Derivative assets	12	4
Short-term investments	1,878	3,244
Total assets accounted for at fair value on a recurring basis	42,488	37,071
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(63)	(69)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(63)	(69)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	11	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(1)	(2)
Significant Observable Inputs (Level 2) | Equity 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	1
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets		(2)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(2)	(5)
Significant Observable Inputs (Level 2) | Interest rate derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	1
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(60)	(63)
Significant Observable Inputs (Level 2) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,461	1,266
Significant Observable Inputs (Level 2) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	2,088	1,337
Significant Observable Inputs (Level 2) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	4,329	3,540
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	16,664	12,878
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,120	844
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	9,498	10,346
Significant Observable Inputs (Level 2) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	4,309	2,359
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	935	1,187
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	1,414	1,565
Fixed maturities, FVO	0	0
Equity securities, at fair value	73	77
Derivative assets	0	3
Short-term investments	15	0
Total assets accounted for at fair value on a recurring basis	1,502	1,645
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(15)	1
Contingent consideration	(22)	(35)
Total liabilities accounted for at fair value on a recurring basis	(37)	(34)
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		3
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(15)	0
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	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	1
Significant Unobservable Inputs (Level 3) | Asset backed securities (ABS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	15	10
Significant Unobservable Inputs (Level 3) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	95	100
Significant Unobservable Inputs (Level 3) | Commercial mortgage-backed securities (CMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	9	12
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Total fixed maturities	732	520
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Total fixed maturities	3	3
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Total fixed maturities	0	0
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Total fixed maturities	560	920
Significant Unobservable Inputs (Level 3) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Total fixed maturities	$ 0	$ 0