Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 42,389	$ 35,652
Fixed maturities, FVO	39	22
Equity securities, at fair value	1,414	1,214
Derivative assets	12	7
Short-term investments	2,927	4,283
Total assets accounted for at fair value on a recurring basis	46,781	41,178
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(71)	(68)
Contingent consideration	(21)	(35)
Total liabilities accounted for at fair value on a recurring basis	(92)	(103)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	9	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(1)	(2)
Equity
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	(5)	1
Foreign exchange derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	4	(2)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	4	(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	(69)	(62)
Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,337	1,276
Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,158	1,437
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,254	3,552
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	17,801	13,398
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,117	847
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,895	10,346
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,732	3,279
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,095	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	7	330
Fixed maturities, FVO	0	0
Equity securities, at fair value	1,196	1,093
Derivative assets	0	0
Short-term investments	1,211	1,039
Total assets accounted for at fair value on a recurring basis	2,414	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
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	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	7	330
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	40,726	33,757
Fixed maturities, FVO	39	22
Equity securities, at fair value	148	44
Derivative assets	12	4
Short-term investments	1,716	3,244
Total assets accounted for at fair value on a recurring basis	42,641	37,071
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(66)	(69)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(66)	(69)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	9	5
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(1)	(2)
Significant Observable Inputs (Level 2) | Equity
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	4	(2)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	4	(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	(69)	(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	1,337	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	1,862	1,337
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,234	3,540
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	17,078	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	1,114	844
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,895	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	4,118	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	1,088	1,187
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,656	1,565
Fixed maturities, FVO	0	0
Equity securities, at fair value	70	77
Derivative assets	0	3
Short-term investments	0	0
Total assets accounted for at fair value on a recurring basis	1,726	1,645
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(5)	1
Contingent consideration	(21)	(35)
Total liabilities accounted for at fair value on a recurring basis	(26)	(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
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	(5)	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	0	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	296	100
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	20	12
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	723	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	614	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
Fair Value Measured at Net Asset Value Per Share [Member]
Liabilities accounted for at fair value on a recurring basis
Deposit Assets	$ 55	$ 0