Reserve for Property and Casualty Insurance Claims and Claims Expense - Prior year reserve reestimates included in claims and claims expense (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Increase (decrease) in claims and claims expense
Prior years	$ (65)	$ (440)
Auto
Increase (decrease) in claims and claims expense
Prior years	3	(101)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(156)	(440)
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(83)	(66)
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	58	29
Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	115	139
Unfavorable reestimates	111
Protection Services
Increase (decrease) in claims and claims expense
Prior years	(2)	(1)
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years	142	62
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years	31	(60)
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	12	(19)
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(69)	(25)
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	55	28
Non-catastrophe losses | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	115	139
Non-catastrophe losses | Protection Services
Increase (decrease) in claims and claims expense
Prior years	(2)	(1)
Catastrophe
Increase (decrease) in claims and claims expense
Prior years	(207)	(502)
Catastrophe | Nationwide Aggregate Reinsurance Program
Increase (decrease) in claims and claims expense
Prior years	240
Catastrophe | PG&E And Southern California Edison
Increase (decrease) in claims and claims expense
Prior years		495
Catastrophe | Auto
Increase (decrease) in claims and claims expense
Prior years	(28)	(41)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(168)	(421)
Catastrophe | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(14)	(41)
Catastrophe | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	3	1
Catastrophe | Run-off Property-Liability
Increase (decrease) in claims and claims expense
Prior years	0	0
Catastrophe | Protection Services
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