Reinsurance (Consolidated Reinsurance) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums written [Abstract]
Direct	$ 37,749	$ 36,848	$ 34,782
Assumed	3,512	3,276	3,186
Ceded	(7,441)	(7,849)	(7,389)
Net	33,820	32,275	30,579
Premiums earned [Abstract]
Direct	37,037	35,876	34,108
Assumed	3,323	3,107	3,175
Ceded	(7,243)	(7,693)	(7,219)
Net premiums earned	33,117	31,290	30,064
Reinsurance recoveries on losses and loss expenses incurred	$ 5,000	$ 4,900	$ 5,600