Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 44,044	$ 42,148
Fixed maturities, FVO	0	11
Equity securities, at fair value	819	1,657
Derivative assets	12	12
Short-term investments	3,399	2,921
Total assets accounted for at fair value on a recurring basis	48,274	46,749
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(72)	(78)
Contingent consideration		(22)
Total liabilities accounted for at fair value on a recurring basis	(72)	(100)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	11	11
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	1	(1)
Equity
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(15)
Foreign exchange derivatives
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	6	(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	(79)	(60)
Asset-backed-securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,490	1,476
Collateralized loan obligations ("CLOs")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,449	2,183
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,444	4,338
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	19,416	17,396
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	984	1,123
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,310	9,498
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,548	4,869
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,403	1,265
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	381	330
Fixed maturities, FVO		0
Equity securities, at fair value	352	1,401
Derivative assets	0	0
Short-term investments	3,176	1,028
Total assets accounted for at fair value on a recurring basis	3,909	2,759
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Contingent consideration		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
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
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) | 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	381	330
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	41,977	40,404
Fixed maturities, FVO		11
Equity securities, at fair value	398	183
Derivative assets	12	12
Short-term investments	209	1,878
Total assets accounted for at fair value on a recurring basis	42,596	42,488
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(72)	(63)
Contingent consideration		0
Total liabilities accounted for at fair value on a recurring basis	(72)	(63)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	11	11
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	1	(1)
Significant Observable Inputs (Level 2) | Foreign exchange derivatives
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	6	(2)
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	(79)	(60)
Significant Observable Inputs (Level 2) | Asset-backed-securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,490	1,461
Significant Observable Inputs (Level 2) | Collateralized loan obligations ("CLOs")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,308	2,088
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,352	4,329
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	18,420	16,664
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	984	1,120
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,304	9,498
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,097	4,309
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,022	935
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,686	1,414
Fixed maturities, FVO		0
Equity securities, at fair value	69	73
Derivative assets	0	0
Short-term investments	14	15
Total assets accounted for at fair value on a recurring basis	1,769	1,502
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	(15)
Contingent consideration		(22)
Total liabilities accounted for at fair value on a recurring basis	0	(37)
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
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities		(15)
Significant Unobservable Inputs (Level 3) | Foreign exchange derivatives
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	0
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	15
Significant Unobservable Inputs (Level 3) | Collateralized loan obligations ("CLOs")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	141	95
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	92	9
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	996	732
Significant Unobservable Inputs (Level 3) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	0	3
Significant Unobservable Inputs (Level 3) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	6	0
Significant Unobservable Inputs (Level 3) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	451	560
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	$ 53	$ 38