Reserve for Property and Casualty 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, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (decrease) in claims and claims expense
Incurred	$ 22,001	$ 23,976	$ 22,778
Paid	(22,224)	(23,444)	(22,364)
Claims incurred and paid from before 2016
Increase (decrease) in claims and claims expense
Incurred	(32)
Paid	(423)
Auto insurance - liability coverage
Increase (decrease) in claims and claims expense
Incurred	7,629
Paid	(7,939)
Auto insurance - physical damage coverage
Increase (decrease) in claims and claims expense
Incurred	4,817
Paid	(4,716)
Homeowners
Increase (decrease) in claims and claims expense
Incurred	4,889
Paid	(4,731)
Total auto and homeowners insurance
Increase (decrease) in claims and claims expense
Incurred	17,335
Paid	(17,386)
Other personal lines
Increase (decrease) in claims and claims expense
Incurred	1,015
Paid	(1,007)
Commercial lines
Increase (decrease) in claims and claims expense
Incurred	585
Paid	(463)
Protection Services
Increase (decrease) in claims and claims expense
Incurred	319
Paid	(326)
Unallocated loss adjustment expenses (“ULAE”)
Increase (decrease) in claims and claims expense
Incurred	2,681
Paid	(2,590)
Other
Increase (decrease) in claims and claims expense
Incurred	(34)
Paid	59
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Incurred	132
Paid	(88)
Catastrophe losses
Increase (decrease) in claims and claims expense
Incurred	$ 2,810	$ 2,560	$ 2,860