Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	$ 43,607	$ 45,035
Equity securities, at fair value	1,632	1,438
Derivative assets	22	23
Short-term investments	3,367	3,283
Total assets accounted for at fair value on a recurring basis	48,628	49,779
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(55)	(84)
Total liabilities accounted for at fair value on a recurring basis	(55)	(84)
Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	21	21
Foreign exchange 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	(10)	(14)
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	(45)	(70)
Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,435	1,564
Collateralized loan obligations ("CLOs")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,049	2,780
CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,167	4,484
Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	19,495	20,273
Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	868	919
Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,214	9,503
RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,025	4,107
U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,354	1,405
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	547	529
Equity securities, at fair value	1,022	872
Derivative assets	0	0
Short-term investments	1,754	2,663
Total assets accounted for at fair value on a recurring basis	3,323	4,064
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	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
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
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	547	529
Significant Observable Inputs (Level 2)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	41,142	42,801
Equity securities, at fair value	540	496
Derivative assets	22	23
Short-term investments	1,597	590
Total assets accounted for at fair value on a recurring basis	43,301	43,910
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	(55)	(84)
Total liabilities accounted for at fair value on a recurring basis	(55)	(84)
Significant Observable Inputs (Level 2) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	21	21
Significant Observable Inputs (Level 2) | Foreign exchange 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	(10)	(14)
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	(45)	(70)
Significant Observable Inputs (Level 2) | Asset backed securities ("ABS")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,429	1,564
Significant Observable Inputs (Level 2) | Collateralized loan obligations ("CLOs")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	2,623	2,420
Significant Observable Inputs (Level 2) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	4,106	4,407
Significant Observable Inputs (Level 2) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	18,551	19,392
Significant Observable Inputs (Level 2) | Foreign government/government agencies
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	868	913
Significant Observable Inputs (Level 2) | Municipal
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	9,214	9,503
Significant Observable Inputs (Level 2) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	3,544	3,726
Significant Observable Inputs (Level 2) | U.S. Treasuries
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	807	876
Significant Unobservable Inputs (Level 3)
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	1,918	1,705
Equity securities, at fair value	70	70
Derivative assets	0	0
Short-term investments	16	30
Total assets accounted for at fair value on a recurring basis	2,004	1,805
Liabilities accounted for at fair value on a recurring basis
Derivative liabilities	0	0
Total liabilities accounted for at fair value on a recurring basis	0	0
Significant Unobservable Inputs (Level 3) | Credit derivatives
Assets accounted for at fair value on a recurring basis
Derivative assets	0	0
Significant Unobservable Inputs (Level 3) | Foreign exchange 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
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	6	0
Significant Unobservable Inputs (Level 3) | Collateralized loan obligations ("CLOs")
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	426	360
Significant Unobservable Inputs (Level 3) | CMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	61	77
Significant Unobservable Inputs (Level 3) | Corporate
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	944	881
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	6
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) | RMBS
Assets accounted for at fair value on a recurring basis
Debt Securities, Available-for-sale	481	381
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	$ 61	$ 54