Reserve for Property and Liability Insurance Claims and Claims Expense - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Increase (decrease) in claims and claims expense
Reserves for asbestos claims	$ 856		$ 884
Reinsurance recoverables for asbestos claims	392		412
Reserves for environmental claims	159		166
Reinsurance recoverables for environmental claims	36		$ 33
Prior years claims and claims adjust expense, favorable (unfavorable)	192
Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	225
Catastrophe
Increase (decrease) in claims and claims expense
Total incurred	1,270	$ 1,750
Prior years claims and claims adjust expense, favorable (unfavorable)	(44)
Reserve Adjustments | Other personal lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(33)
Catastrophe Loss Re-estimates
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(44)
Non Catastrophe Reestimates | Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	34
Non Catastrophe Reestimates | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(78)
Hurricane | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	$ (37)