Reserve for Property and Casualty Insurance Claims and Claims Expense - Reserves for asbestos, environmental and other discontinued lines claims before and after the effects of reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Asbestos claims
Gross reserves	$ 1,204	$ 1,172
Reinsurance	(377)	(362)
Net reserves	827	810
Environmental claims
Gross reserves	249	219
Reinsurance	(43)	(40)
Net reserves	206	179
Other discontinued lines
Gross reserves	435	427
Reinsurance	(60)	(51)
Net reserves	375	376
Total
Gross reserves	1,888	1,818
Reinsurance	(480)	(453)
Net reserves	$ 1,408	$ 1,365