Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 36,819	$ 35,652
Fixed maturities, FVO	20	22
Equity securities, at fair value	1,275	1,214
Derivative assets	24	7
Short-term investments	4,203	4,283
Total assets accounted for at fair value on a recurring basis	42,341	41,178
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(64)	(68)
Contingent consideration	(29)	(35)
Total liabilities accounted for at fair value on a recurring basis	(93)	(103)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	24	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(5)	(2)
Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	3
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		1
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	(2)	(2)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(3)	(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	(56)	(62)
Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	968	1,276
Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,438	1,437
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,568	3,552
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	14,403	13,398
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	882	847
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	10,346	10,346
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,548	3,279
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,666	1,517
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	122	330
Fixed maturities, FVO	0	0
Equity securities, at fair value	1,161	1,093
Short-term investments	1,488	1,039
Total assets accounted for at fair value on a recurring basis	2,771	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	0
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		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) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	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
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	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
Debt Securities, Available-for-sale	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	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
Debt Securities, Available-for-sale	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
Debt Securities, Available-for-sale	122	330
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	35,263	33,757
Fixed maturities, FVO	20	22
Equity securities, at fair value	41	44
Derivative assets	23	4
Short-term investments	2,715	3,244
Total assets accounted for at fair value on a recurring basis	38,062	37,071
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(64)	(69)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(64)	(69)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	24	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(5)	(2)
Significant Observable Inputs (Level 2) | Equity 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
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	(2)	(2)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(3)	(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	(56)	(63)
Significant Observable Inputs (Level 2) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	959	1,266
Significant Observable Inputs (Level 2) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,324	1,337
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,556	3,540
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	13,878	12,878
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	879	844
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	10,346	10,346
Significant Observable Inputs (Level 2) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,777	2,359
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,544	1,187
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,434	1,565
Fixed maturities, FVO	0	0
Equity securities, at fair value	73	77
Derivative assets	1	3
Short-term investments	0	0
Total assets accounted for at fair value on a recurring basis	1,508	1,645
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	1
Contingent consideration	(29)	(35)
Total liabilities accounted for at fair value on a recurring basis	(29)	(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	1	3
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		0
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	0	1
Significant Unobservable Inputs (Level 3) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9	10
Significant Unobservable Inputs (Level 3) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	114	100
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	12	12
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	525	520
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3	3
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	0
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	771	920
Significant Unobservable Inputs (Level 3) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 0	$ 0