Reserve for Property and Liability Insurance Claims and Claims Expense - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Increase (decrease) in claims and claims expense
Reserves for asbestos claims	$ 882		$ 884
Reinsurance recoverables for asbestos claims	423		412
Reserves for environmental claims	174		166
Reinsurance recoverables for environmental claims	40		$ 33
Prior years claims and claims adjust expense, favorable (unfavorable)	135
Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	372
Catastrophe
Increase (decrease) in claims and claims expense
Total incurred	1,890	$ 2,640
Prior years claims and claims adjust expense, favorable (unfavorable)	(45)
Hurricane | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(37)
Reserve Adjustments | Other personal lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(192)
Reserve Adjustments | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(85)
Reserve Adjustments | Commercial lines
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	(107)
Non Catastrophe Reestimates
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	180
Non Catastrophe Reestimates | Auto
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	39
Non Catastrophe Reestimates | Homeowners
Increase (decrease) in claims and claims expense
Prior years claims and claims adjust expense, favorable (unfavorable)	$ (84)