Reserve for Property and Casualty Insurance Claims and Claims Expense - Prior year reserve reestimates included in claims and claims expense (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Increase (decrease) in claims and claims expense
Prior years	$ (241)	$ 8
Auto
Increase (decrease) in claims and claims expense
Prior years	(36)	13
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(203)	(7)
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(18)	(6)
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	15	6
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	1	2
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	2	28
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(17)	22
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	5	1
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	0	(2)
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	13	5
Non-catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	1	2
Catastrophe
Increase (decrease) in claims and claims expense
Prior years	(243)	(20)
Catastrophe | Nationwide Aggregate Reinsurance Program
Increase (decrease) in claims and claims expense
Prior years	150
Catastrophe | Auto
Increase (decrease) in claims and claims expense
Prior years	(19)	(9)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(208)	(8)
Catastrophe | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(18)	(4)
Catastrophe | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	2	1
Catastrophe | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	0	$ 0
Fire | Southern California Edison
Increase (decrease) in claims and claims expense
Prior years	$ 110