Reserve for Property and Liability Insurance Claims and Claims Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Increase (decrease) in claims and claims expense
Reserves for asbestos claims	$ 847		$ 866
Reinsurance recoverables for asbestos claims	389		400
Reserves for environmental claims	167		170
Reinsurance recoverables for environmental claims	39		$ 39
Prior years claims and claims adjust expense, favorable (unfavorable)	(12)
Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	55
Catastrophe
Increase (decrease) in claims and claims expense
Total incurred	680	$ 361
Prior years claims and claims adjust expense, favorable (unfavorable)	(53)
Catastrophe | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(15)
Reserve Adjustments | Other personal lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(14)
Reserve Adjustments | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(2)
Reserve Adjustments | Commercial lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(5)
Reserve Adjustments | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(7)
Non Catastrophe Reestimates
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	41
Non Catastrophe Reestimates | Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(7)
Non Catastrophe Reestimates | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	$ (46)