Reportable Segments - Reportable segments financial performance (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax									$ 3,425	$ 1,068	$ 1,630
Net investment income									3,293	1,590	1,728
Net gains (losses) on investments and derivatives									1,084	1,087	1,538
Pension and other postretirement remeasurement gains (losses)									644	51	(114)
Curtailment gains (losses)									0	8	0
Business combination expenses and amortization of purchased intangibles									(157)	(106)	(122)
Business combination fair value adjustment									6	0	0
Impairment of purchased intangibles									0	0	(55)
Income tax (expense) benefit on reconciling items									(1,270)	(1,393)	(1,150)
(Loss) income from discontinued operations									(3,612)	157	646
Income tax benefit (expense) from discontinued operations									19	(10)	(126)
(Loss) income from discontinued operations, net of tax	$ (321)	$ 325	$ 196	$ (3,793)	$ 354	$ (63)	$ 144	$ (288)	(3,593)	147	520
Less: Net loss attributable to noncontrolling interest									(33)	0	0
Net income applicable to common shareholders	$ 790	$ 508	$ 1,595	$ (1,408)	$ 2,598	$ 1,126	$ 1,224	$ 513	1,485	5,461	4,678
Allstate Protection
Segment Reporting Information
Underwriting income									1,785	4,569	2,921
Run-off Property-Liability
Segment Reporting Information
Underwriting income									(120)	(144)	(108)
Property-Liability
Segment Reporting Information
Underwriting income									1,665	4,425	2,813
Net investment income									3,118	1,421	1,533
Net gains (losses) on investments and derivatives									1,021	990	1,470
Less: Net loss attributable to noncontrolling interest									(34)	0	0
Protection Services
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax									179	153	38
Net investment income									43	44	42
Net gains (losses) on investments and derivatives									25	30	32
Allstate Health and Benefits
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax									208	96	115
Net investment income									74	78	83
Net gains (losses) on investments and derivatives									7	8	12
Corporate and Other
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax									$ (433)	$ (428)	$ (438)