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
	Mar. 31, 2020	Mar. 31, 2019
Increase (decrease) in claims and claims expense
Prior years	$ 8	$ 12
Auto
Increase (decrease) in claims and claims expense
Prior years	13	(54)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	(7)	53
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(6)	7
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	6	4
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years	2	2
Service Businesses
Increase (decrease) in claims and claims expense
Prior years	0	0
Non-catastrophes losses
Increase (decrease) in claims and claims expense
Prior years	28	(41)
Non-catastrophes losses | Auto
Increase (decrease) in claims and claims expense
Prior years	22	(53)
Non-catastrophes losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	1	7
Non-catastrophes losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(2)	(2)
Non-catastrophes losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	5	5
Non-catastrophes losses | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years	2	2
Non-catastrophes losses | Service Businesses
Increase (decrease) in claims and claims expense
Prior years	0	0
Catastrophe
Increase (decrease) in claims and claims expense
Prior years	(20)	53
Catastrophe | Auto
Increase (decrease) in claims and claims expense
Prior years	(9)	(1)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(8)	46
Reinstatement reinsurance premiums		15
Catastrophe | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(4)	9
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