Reportable Segments - Total assets and investments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Assets	$ 125,987	$ 119,950
Investments	94,237	88,362
Deferred policy acquisition costs	4,700	4,699	$ 4,784	$ 4,191
Property-Liability
Segment Reporting Information
Assets	69,171	67,243
Investments	50,134	48,414
Deferred policy acquisition costs	1,608	1,624
Protection Services
Segment Reporting Information
Assets	6,177	5,746
Investments	1,822	1,544
Deferred policy acquisition costs	1,696	1,448
Allstate Life
Segment Reporting Information
Assets	15,051	14,771
Investments	12,406	11,914
Deferred policy acquisition costs	909	1,079
Allstate Benefits
Segment Reporting Information
Assets	2,905	2,915
Investments	2,012	1,941
Deferred policy acquisition costs	470	527
Allstate Annuities
Segment Reporting Information
Assets	27,080	26,914
Investments	22,291	22,221
Deferred policy acquisition costs	17	21
Corporate and Other
Segment Reporting Information
Assets	5,603	2,361
Investments	$ 5,572	$ 2,328