Reserve for Property-Liability Insurance Claims and Claims Expense - Incurred and claims expense narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	$ 23,976	$ 22,778	$ 21,847
Net increase (decrease) to reserves	130	255	503
Auto
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	323	455	490
Homeowners
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	4,625
Other personal lines
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	1,024
Net increase (decrease) to reserves	(8)	7	(1)
Commercial lines
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	648
Net increase (decrease) to reserves	(17)	(108)	(19)
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Unfavorable reestimate	95
Commercial Lines, Discontinued Lines and Coverages and Other Personal Lines [Member]
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	2	2	(2)
Catastrophes
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	2,560	2,860	3,230
Catastrophes | Auto
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	17	39	15
Catastrophes | Homeowners
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(66)	(65)	7
Catastrophes | Other personal lines
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	0	1	(3)
Catastrophes | Commercial lines
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	1	0	(1)
Catastrophes | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	0	0	0
Catastrophes | Commercial Lines, Discontinued Lines and Coverages and Other Personal Lines [Member]
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	0	0	0
Catastrophe loss reestimates
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(48)	(25)	18
Reserve reestimates | Homeowners
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(65)	(14)	131
Reserve reestimates | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(105)	(87)	(96)
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Losses from catastrophes, net of recoveries	91
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	178	280	485
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	306	416	475
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	1	51	124
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(8)	6	2
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(18)	(108)	(18)
Non-catastrophe losses | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	(105)	(87)	(96)
Non-catastrophe losses | Commercial Lines, Discontinued Lines and Coverages and Other Personal Lines [Member]
Increase (decrease) in claims and claims expense
Net increase (decrease) to reserves	$ 2	$ 2	$ (2)