Claim and Claim Adjustment Expense Reserves - Reconciliation of Claim and Claim Adjustment Expense Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross reserves, beginning of year	$ 22,120	$ 21,269	$ 19,862
Ceded reserves, beginning of year	5,191	4,969	4,005
Net reserves, beginning of year	16,929	16,300	15,857
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	5,667	5,181	5,021
Increase (decrease) in provision for insured events of prior years	48	(32)	15
Amortization of discount	44	44	48
Total net incurred	5,759	5,193	5,084
Net payments attributable to:
Current year events	(922)	(821)	(933)
Prior year events	(3,679)	(3,481)	(3,016)
Total net payments	(4,601)	(4,302)	(3,949)
Foreign currency translation adjustment and other	76	(262)	(60)
Net reserves, end of year	18,163	16,929	16,300
Ceded reserves, end of year	5,141	5,191	4,969
Gross reserves, end of year	$ 23,304	$ 22,120	$ 21,269