Reserve for Property-Liability Insurance Claims and Claims Expense - Incurred and claims expense narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Auto
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	$ (490)	$ (155)	$ 30
Catastrophes
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	3,230	2,570	1,720
Non-catastrophes loss reestimates | Homeowners
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(131)	(24)
Catastrophe loss reestimates
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	18	6	(15)
Catastrophe loss reestimates | Homeowners
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves			(24)
Reserve reestimates | Other personal lines
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	22	57	22
Reserve reestimates | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves		$ 105	$ 53
Discontinued Lines and Coverages | Catastrophe loss reestimates
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	$ 96