Reportable Segments - Reportable segments financial performance (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information
Net investment income									$ 2,853	$ 3,159	$ 3,240
Net income applicable to common shareholders	$ 2,598	$ 1,126	$ 1,224	$ 513	$ 1,707	$ 889	$ 821	$ 1,261	5,461	4,678	2,012
Allstate Protection
Segment Reporting Information
Underwriting income									4,566	2,912	2,343
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income									(144)	(108)	(90)
Property-Liability
Segment Reporting Information
Underwriting income									4,422	2,804	2,253
Net investment income									1,421	1,533	1,464
Income tax expense on operations									(1,166)	(887)	(747)
Realized capital gains (losses), after-tax									774	1,161	(500)
Tax Legislation expense (benefit)									0	0	(5)
Net income applicable to common shareholders									5,451	4,611	2,465
Protection Services
Segment Reporting Information
Net investment income									44	42	27
Adjusted net income									153	38	8
Realized capital gains (losses), after-tax									23	25	(9)
Amortization of purchased intangibles, after-tax									(84)	(97)	(74)
Impairment of purchased intangibles, after-tax									0	(43)	0
Tax Legislation expense (benefit)									0	0	(4)
Net income applicable to common shareholders									92	(77)	(79)
Allstate Life
Segment Reporting Information
Net investment income									502	514	505
Adjusted net income									194	261	295
Realized capital gains (losses), after-tax									(9)	0	(11)
Valuation changes on embedded derivatives that are not hedged, after-tax									(34)	(9)	0
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax									8	(5)	(8)
Tax Legislation expense (benefit)									0	0	(16)
Net income applicable to common shareholders									159	247	260
Allstate Benefits
Segment Reporting Information
Net investment income									78	83	77
Adjusted net income									96	115	124
Realized capital gains (losses), after-tax									7	9	(7)
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax									0	0	1
Tax Legislation expense (benefit)									0	0	0
Net income applicable to common shareholders									103	124	118
Allstate Annuities
Segment Reporting Information
Net investment income									761	917	1,096
Adjusted net income									(53)	10	131
Realized capital gains (losses), after-tax									221	274	(131)
Valuation changes on embedded derivatives that are not hedged, after-tax									(2)	(6)	3
Premium deficiency for immediate annuities, after tax									(178)	0	0
Gain on disposition of operations, after-tax									3	4	4
Tax Legislation expense (benefit)									0	0	69
Net income applicable to common shareholders									(9)	282	76
Allstate Annuities | Reinsurance ceded
Segment Reporting Information
Premium deficiency for immediate annuities, after tax		$ 225
Corporate and Other
Segment Reporting Information
Net investment income									47	70	71
Adjusted net income									(428)	(438)	(406)
Realized capital gains (losses), after-tax									47	19	(30)
Pension and other postretirement remeasurement gains (losses), after-tax									39	(90)	(370)
Curtailment gain, after-tax									7	0	0
Business combination expenses, after-tax									0	0	(7)
Tax Legislation expense (benefit)									0	0	(15)
Net income applicable to common shareholders									$ (335)	$ (509)	$ (828)