Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	$ 36,194	$ 36,964
Fixed maturities, FVO	36	41
Equity securities, at fair value	1,003	0
Equity securities, AFS	0	1,012
Derivative assets	15	10
Short-term investments	3,296	2,270
Total assets accounted for at fair value on a recurring basis	40,544	40,297
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(75)	(100)
Contingent consideration	(31)	(29)
Total liabilities accounted for at fair value on a recurring basis	(106)	(129)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	15	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(5)	(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	(11)	(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	(59)	(84)
Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	994	1,126
Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,089	1,260
CMBS
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	3,494	3,336
Corporate
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	13,349	12,804
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,133	1,110
Municipal
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	11,142	12,485
Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	3,207	3,044
Fixed maturities, FVO	36	41
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,786	1,799
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	378	333
Fixed maturities, FVO	0	0
Equity securities, at fair value	892
Equity securities, AFS		887
Short-term investments	861	1,098
Total assets accounted for at fair value on a recurring basis	2,131	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	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
Available-for-sale Securities, Debt Securities	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
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | CMBS
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	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
Available-for-sale Securities, Debt Securities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	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
Available-for-sale Securities, Debt Securities	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
Available-for-sale Securities, Debt Securities	378	333
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	33,864	34,679
Fixed maturities, FVO	36	41
Equity securities, at fair value	45
Equity securities, AFS		49
Derivative assets	14	9
Short-term investments	2,435	1,172
Total assets accounted for at fair value on a recurring basis	36,394	35,950
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(77)	(101)
Contingent consideration	0	0
Total liabilities accounted for at fair value on a recurring basis	(77)	(101)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	15	9
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(5)	(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	(11)	(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	(61)	(85)
Significant Observable Inputs (Level 2) | Asset-backed-securities (ABS)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	937	1,107
Significant Observable Inputs (Level 2) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	930	1,165
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	3,466	3,267
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	12,790	12,284
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,130	1,108
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	11,133	12,468
Significant Observable Inputs (Level 2) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	2,070	1,814
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,408	1,466
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,952	1,952
Fixed maturities, FVO	0	0
Equity securities, at fair value	66
Equity securities, AFS		76
Derivative assets	1	1
Short-term investments	0	0
Total assets accounted for at fair value on a recurring basis	2,019	2,029
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	2	1
Contingent consideration	(31)	(29)
Total liabilities accounted for at fair value on a recurring basis	(29)	(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	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
Available-for-sale Securities, Debt Securities	57	19
Significant Unobservable Inputs (Level 3) | Collateralized debt obligations (CDOs)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	159	95
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	28	69
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	559	520
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	3	2
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	9	17
Significant Unobservable Inputs (Level 3) | Residential mortgage-backed securities (RMBS)
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	1,137	1,230
Significant Unobservable Inputs (Level 3) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Available-for-sale Securities, Debt Securities	$ 0	$ 0