Benefit Plans - Allstate 401(k) savings plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Retirement Benefits [Abstract]
Principal balance of note from ESOP	$ 2,000,000
Company's contribution to the Allstate Plan	93,000,000	$ 89,000,000	$ 81,000,000
ESOP benefit computed
Interest expense recognized by ESOP	0	0	0
Less: dividends accrued on ESOP shares	(1,000,000)	(1,000,000)	(1,000,000)
Cost of shares allocated	3,000,000	0	3,000,000
Compensation expense	2,000,000	(1,000,000)	2,000,000
Reduction of defined contribution due to ESOP	43,000,000	1,000,000	38,000,000
ESOP benefit	(41,000,000)	(2,000,000)	(36,000,000)
Contributions to ESOP	$ 2,000,000	0	1,000,000
Number of committed-to-be-released (in shares)	400,000
Allocated ESOP shares (in shares)	39,000,000
Unallocated ESOP shares (in shares)	0
Defined contribution plan expense for eligible employees of Canadian insurance subsidiaries and Sterling	$ 15,000,000	$ 15,000,000	$ 12,000,000