Reserve for Property and Casualty Insurance Claims and Claims Expense 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	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019
Increase (decrease) in claims and claims expense
Prior years		$ (440)	$ (118)
Auto
Increase (decrease) in claims and claims expense
Prior years		(101)	(311)
Homeowners
Increase (decrease) in claims and claims expense
Prior years		(440)	59
Other personal lines
Increase (decrease) in claims and claims expense
Prior years		(66)	17
Commercial lines
Increase (decrease) in claims and claims expense
Prior years		29	17
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years		139	103
Service Businesses
Increase (decrease) in claims and claims expense
Prior years		(1)	(3)
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years		62	(171)
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years		(60)	(302)
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years		(19)	0
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years		(25)	13
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years		28	18
Non-catastrophe losses | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years		139	103
Non-catastrophe losses | Service Businesses
Increase (decrease) in claims and claims expense
Prior years		(1)	(3)
Annual reserve review | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years		132
Catastrophe
Increase (decrease) in claims and claims expense
Prior years	$ (495)	(502)	53
Catastrophe | Auto
Increase (decrease) in claims and claims expense
Prior years		(41)	(9)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years		(421)	59
Catastrophe | Other personal lines
Increase (decrease) in claims and claims expense
Prior years		(41)	4
Catastrophe | Commercial lines
Increase (decrease) in claims and claims expense
Prior years		1	(1)
Catastrophe | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years		0	0
Catastrophe | Service Businesses
Increase (decrease) in claims and claims expense
Prior years		$ 0	$ 0