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	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Increase (decrease) in claims and claims expense
Prior years	$ (3)	$ (74)
Auto
Increase (decrease) in claims and claims expense
Prior years	(41)	(158)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	16	57
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(8)	8
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	27	17
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years	4	5
Service Businesses
Increase (decrease) in claims and claims expense
Prior years	(1)	(3)
Non-catastrophes losses
Increase (decrease) in claims and claims expense
Prior years	(8)	(130)
Non-catastrophes losses | Auto
Increase (decrease) in claims and claims expense
Prior years	(26)	(151)
Non-catastrophes losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years	(3)	0
Non-catastrophes losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(6)	2
Non-catastrophes losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	24	17
Non-catastrophes losses | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years	4	5
Non-catastrophes losses | Service Businesses
Increase (decrease) in claims and claims expense
Prior years	(1)	(3)
Catastrophe
Increase (decrease) in claims and claims expense
Prior years	5	56
Catastrophe | Auto
Increase (decrease) in claims and claims expense
Prior years	(15)	(7)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years	19	57
Catastrophe | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	(2)	6
Catastrophe | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	3	0
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