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
Reserves, beginning of year:
Gross	$ 22,120	$ 21,269	$ 19,862
Ceded	5,191	4,969	4,005
Net reserves, beginning of year	16,929	16,300	15,857
Reduction of net reserves due to Excess Workers' Compensation Loss Portfolio Transfer	0	0	(632)
Net incurred claim and claim adjustment expenses:
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