Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Reserves, beginning of year:
Gross	$ 24,174	$ 22,706
Ceded	4,969	4,005
Net reserves, beginning of year	19,205	18,701
Reduction of net reserves due to Excess Workers' Compensation Loss Portfolio Transfer	0	(632)
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	1,448	1,474
Increase (decrease) in provision for insured events of prior years	(45)	(54)
Amortization of discount	47	50
Total net incurred	1,450	1,470
Net payments attributable to:
Current year events	(70)	(85)
Prior year events	(1,147)	(1,067)
Total net payments	(1,217)	(1,152)
Foreign currency translation adjustment and other	(92)	(32)
Net reserves, end of period	19,346	18,355
Ceded reserves, end of period	5,002	4,701
Gross reserves, end of period	$ 24,348	$ 23,056