Reserve for Property and Liability Insurance Claims and Claims Expense - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Increase (decrease) in claims and claims expense
Reserves for asbestos claims	$ 826		$ 866
Reinsurance recoverables for asbestos claims	381		400
Reserves for environmental claims	155		170
Reinsurance recoverables for environmental claims	36		$ 39
Prior years claims and claims adjust expense, favorable (unfavorable)	(74)
Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(154)
Catastrophe
Increase (decrease) in claims and claims expense
Total incurred	1,750	$ 1,270
Prior years claims and claims adjust expense, favorable (unfavorable)	56
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	20
Reserve Adjustments | Commercial Lines, Discontinued Lines and Coverages and Other Personal Lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	24
Reserve Adjustments | Other personal lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	2
Reserve Adjustments | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	5
Reserve Adjustments | Commercial lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	17
Non Catastrophe Reestimates
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(130)
Non Catastrophe Reestimates | Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(7)
Non Catastrophe Reestimates | Other personal lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	6
Non Catastrophe Reestimates | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	$ 57