Claim and Claim Adjustment Expense Reserves - Reconciliation of Claim and Claim Adjustment Expense Reserves (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	$ 24,174	$ 22,706
Ceded reserves, beginning of year	4,969	4,005
Net reserves, beginning of year	19,205	18,701
Net incurred claim and claim adjustment expenses [Abstract]
Reduction of net reserves due to the excess workers’ compensation loss portfolio transfer		(632)
Provision for insured events of current year	2,974	2,930
Decrease in provision for insured events of prior years	(69)	(78)
Amortization of discount	90	95
Total net incurred	2,995	2,947
Net payments attributable to [Abstract]
Current year events	(245)	(317)
Prior year events	(2,330)	(1,949)
Total net payments	(2,575)	(2,266)
Foreign currency translation adjustment and other	(222)	(5)
Net reserves, end of year	19,403	18,745
Ceded reserves, end of year	5,156	4,735
Gross reserves, end of year	$ 24,559	$ 23,480