Reserve for Property-Liability Insurance Claims and Claims Expense - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Increase (decrease) in claims and claims expense
Reserves for asbestos claims	$ 866		$ 884
Reinsurance recoverables for asbestos claims	399		412
Reserves for environmental claims	162		166
Reinsurance recoverables for environmental claims	33		$ 33
Prior years claims and claims adjust expense, favorable (unfavorable)	51
Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	73
Catastrophe
Increase (decrease) in claims and claims expense
Total incurred	361	$ 781
Non Catastrophe Reestimates | Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	30
Non Catastrophe Reestimates | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(34)
Reserve Adjustments | Other personal lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(18)
Catastrophe Loss Re-estimates
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	$ 4