Reserve for Property and Casualty Insurance Claims and Claims Expense - Incurred and claims expense (Details) $ in Millions			3 Months Ended		12 Months Ended
	Sep. 14, 2020 USD ($)	Jul. 01, 2020 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($) settlement	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jan. 22, 2021 USD ($)	Jun. 20, 2020 USD ($)
Increase (decrease) in claims and claims expense
Prior years					$ (436)	$ (130)	$ (255)
Reinsurance expected recovery percentage		80.00%
Auto
Increase (decrease) in claims and claims expense
Prior years					(107)	(323)	(455)
Homeowners
Increase (decrease) in claims and claims expense
Prior years					(439)	65	14
Other personal lines
Increase (decrease) in claims and claims expense
Prior years					(66)	8	(7)
Commercial lines
Increase (decrease) in claims and claims expense
Prior years					36	17	108
Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years					141	105	87
Protection Services
Increase (decrease) in claims and claims expense
Prior years					(1)	(2)	(2)
Catastrophe losses
Increase (decrease) in claims and claims expense
Prior years				$ (495)	(503)	48	25
Number of settlements | settlement				2
Catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years					(44)	(17)	(39)
Catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years					(422)	66	65
Catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years					(39)	0	(1)
Catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years					2	(1)	0
Catastrophe losses | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years					0	0	0
Catastrophe losses | Protection Services
Increase (decrease) in claims and claims expense
Prior years					0	0	0
Fire | Pacific Gas And Electric
Increase (decrease) in claims and claims expense
Prior years		$ (450)
Catastrophe reinsurance settlement amount									$ 11,000
Fire | Southern California Edison
Increase (decrease) in claims and claims expense
Prior years	$ (45)
Catastrophe reinsurance settlement amount	$ 1,160
Fire | Southern California Edison | Forecast
Increase (decrease) in claims and claims expense
Prior years			$ (110)
Fire | Southern California Edison | Subsequent Event
Increase (decrease) in claims and claims expense
Catastrophe reinsurance settlement amount								$ 2,200
Non-catastrophe losses
Increase (decrease) in claims and claims expense
Prior years					67	(178)	(280)
Non-catastrophe losses | Auto
Increase (decrease) in claims and claims expense
Prior years					(63)	(306)	(416)
Non-catastrophe losses | Homeowners
Increase (decrease) in claims and claims expense
Prior years					(17)	(1)	(51)
Non-catastrophe losses | Other personal lines
Increase (decrease) in claims and claims expense
Prior years					(27)	8	(6)
Non-catastrophe losses | Commercial lines
Increase (decrease) in claims and claims expense
Prior years					34	18	108
Non-catastrophe losses | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years					141	105	87
Non-catastrophe losses | Protection Services
Increase (decrease) in claims and claims expense
Prior years					(1)	$ (2)	$ (2)
Annual reserve review | Discontinued Lines and Coverages
Increase (decrease) in claims and claims expense
Prior years					$ 132