Benefit Plans - Allstate 401(k) savings plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Benefits [Abstract]
Company's contribution to the Allstate Plan	$ 103	$ 93	$ 89
ESOP benefit		(41)	(2)
Principal balance of note from ESOP		2
Defined contribution plan expense for eligible employees of Canadian insurance subsidiaries and Sterling	$ 13	$ 15	$ 15