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	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Increase (decrease) in claims and claims expense
Prior years	$ (118)	$ (135)
Auto
Increase (decrease) in claims and claims expense
Prior years	(311)	(356)
Homeowners
Increase (decrease) in claims and claims expense
Prior years	59	46
Other personal lines
Increase (decrease) in claims and claims expense
Prior years	17	(15)
Commercial lines
Increase (decrease) in claims and claims expense
Prior years	17	107
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years	103	85
Unfavorable reestimates		95
Service Businesses
Increase (decrease) in claims and claims expense
Prior years	(3)	(2)
Non-catastrophes
Increase (decrease) in claims and claims expense
Prior years	(171)	(180)
Non-catastrophes | Auto
Increase (decrease) in claims and claims expense
Prior years	(302)	(319)
Non-catastrophes | Homeowners
Increase (decrease) in claims and claims expense
Prior years	0	(38)
Non-catastrophes | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	13	(13)
Non-catastrophes | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	18	107
Non-catastrophes | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years	103	85
Non-catastrophes | Service Businesses
Increase (decrease) in claims and claims expense
Prior years	(3)	(2)
Catastrophe
Increase (decrease) in claims and claims expense
Prior years	53	45
Catastrophe | Auto
Increase (decrease) in claims and claims expense
Prior years	(9)	(37)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years	59	84
Reinstatement reinsurance premiums	13
Catastrophe | Other personal lines
Increase (decrease) in claims and claims expense
Prior years	4	(2)
Catastrophe | Commercial lines
Increase (decrease) in claims and claims expense
Prior years	(1)	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