Claim and Claim Adjustment Expense Reserves (Reconciliation of claim and claim adjustment expense reserves) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Reserves, beginning of year:
Gross	$ 22,706	$ 21,720
Ceded	4,005	3,835
Net reserves, beginning of year	18,701	17,885
Reduction of net reserves due to Excess Workers' Compensation Loss Portfolio Transfer			$ (632)	$ 0
Net incurred claim and claim adjustment expenses:
Provision for insured events of current year	1,474	1,355
Increase (decrease) in provision for insured events of prior years	(54)	(8)
Amortization of discount	50	51
Total net incurred	1,470	1,398
Net payments attributable to:
Current year events	(85)	(72)
Prior year events	(1,067)	(1,218)
Total net payments	(1,152)	(1,290)
Foreign currency translation adjustment and other	(32)	(88)
Net reserves, end of period	18,355	17,905
Ceded reserves, end of period	4,701	3,967
Gross reserves, end of period	$ 23,056	$ 21,872