Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 36,166	$ 36,964
Fixed maturities, FVO	24	41
Equity securities, at fair value	1,035	0
Equity securities, AFS	0	1,012
Derivative assets	14	10
Short-term investments	3,540	2,270
Total assets accounted for at fair value on a recurring basis	40,779	40,297
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(61)	(100)
Contingent consideration	(32)	(29)
Total liabilities accounted for at fair value on a recurring basis	(93)	(129)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	14	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(3)
Equity derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	1	1
Foreign exchange 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	(10)	(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	(51)	(84)
Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,191	1,126
Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,326	1,260
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,657	3,336
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	13,492	12,804
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	952	1,110
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	10,602	12,485
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,118	3,044
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,828	1,799
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	695	333
Fixed maturities, FVO	0	0
Equity securities, at fair value	905
Equity securities, AFS		887
Short-term investments	980	1,098
Total assets accounted for at fair value on a recurring basis	2,580	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
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	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
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	695	333
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	33,602	34,679
Fixed maturities, FVO	24	41
Equity securities, at fair value	52
Equity securities, AFS		49
Derivative assets	13	9
Short-term investments	2,560	1,172
Total assets accounted for at fair value on a recurring basis	36,251	35,950
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(63)	(101)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(63)	(101)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	14	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(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)	(1)
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(10)	(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	(53)	(85)
Significant Observable Inputs (Level 2) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,137	1,107
Significant Observable Inputs (Level 2) | Collateralized loan obligations (CLOs)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,030	1,165
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,635	3,267
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	12,941	12,284
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	949	1,108
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	10,593	12,468
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	2,184	1,814
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,133	1,466
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,869	1,952
Fixed maturities, FVO	0	0
Equity securities, at fair value	78
Equity securities, AFS		76
Derivative assets	1	1
Short-term investments	0	0
Total assets accounted for at fair value on a recurring basis	1,948	2,029
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	2	1
Contingent consideration	(32)	(29)
Total liabilities accounted for at fair value on a recurring basis	(30)	(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
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	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
Debt Securities, Available-for-sale	54	19
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	95
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	22	69
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	551	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	2
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9	17
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	934	1,230
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