Reserve for Property-Liability Insurance Claims and Claims Expense - Reconciliation of total claims and claims expense incurred and paid by coverage (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (decrease) in claims and claims expense
Incurred	$ 23,976	$ 22,778	$ 21,847
Paid	(23,444)	(22,364)	(21,076)
Claims incurred and paid from before 2015
Increase (decrease) in claims and claims expense
Incurred	(97)
Paid	(444)
Auto insurance - liability coverage
Increase (decrease) in claims and claims expense
Incurred	9,142
Paid	(8,419)
Auto insurance - physical damage coverage
Increase (decrease) in claims and claims expense
Incurred	5,576
Paid	(5,570)
Homeowners
Increase (decrease) in claims and claims expense
Incurred	4,625
Paid	(4,616)
Total auto and homeowners insurance
Increase (decrease) in claims and claims expense
Incurred	19,343
Paid	(18,605)
Other personal lines
Increase (decrease) in claims and claims expense
Incurred	1,024
Paid	(1,059)
Commercial lines
Increase (decrease) in claims and claims expense
Incurred	648
Paid	(404)
Service Businesses
Increase (decrease) in claims and claims expense
Incurred	297
Paid	(311)
Unallocated loss adjustment expenses (“ULAE”)
Increase (decrease) in claims and claims expense
Incurred	2,687
Paid	(2,585)
Other
Increase (decrease) in claims and claims expense
Incurred	(17)
Paid	85
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Incurred	91
Paid	(121)
Catastrophes
Increase (decrease) in claims and claims expense
Incurred	$ 2,560	$ 2,860	$ 3,230