Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information
Property-Liability net investment income	$ 610	$ 562	$ 1,185	$ 1,156
Net gains (losses) on investments and derivatives	(151)	(733)	(137)	(1,000)
Pension and other postretirement remeasurement gains (losses)	40	(259)	93	(12)
Amortization of purchased intangibles	(82)	(87)	(163)	(174)
Less: Net loss attributable to noncontrolling interest	(23)	(9)	(24)	(19)
Net loss applicable to common shareholders, Basic	(1,389)	(1,040)	(1,735)	(406)
Segment Reconciling Items
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	695	(189)	1,348	156
Net gains (losses) on investments and derivatives	(151)	(733)	(137)	(1,000)
Pension and other postretirement remeasurement gains (losses)	40	(259)	93	(12)
Amortization of purchased intangibles	(24)	(28)	(48)	(57)
Gain (loss) on disposition	(8)	27	1	11
Non-recurring costs	(90)	0	(90)	0
Income tax benefit on reconciling items	384	298	476	150
Property-Liability
Segment Reporting Information
Property-Liability net investment income	544	506	1,053	1,064
Net gains (losses) on investments and derivatives	(135)	(662)	(123)	(865)
Less: Net loss attributable to noncontrolling interest	(23)	(10)	(24)	(20)
Property-Liability | Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(2,094)	(864)	(3,095)	(584)
Property-Liability | Segment Reconciling Items
Segment Reporting Information
Property-Liability net investment income	544	506	1,053	1,064
Allstate Protection | Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(2,092)	(861)	(3,090)	(579)
Run-off Property-Liability | Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(2)	(3)	(5)	(5)
Protection Services | Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	41	43	75	96
Allstate Health and Benefits
Segment Reporting Information
Property-Liability net investment income	21	16	40	33
Net gains (losses) on investments and derivatives	1	(12)	3	(19)
Allstate Health and Benefits | Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	57	67	113	124
Corporate and Other | Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	(111)	(107)	(200)	(218)
Property-Liability net investment income	27	28	58	38
Net gains (losses) on investments and derivatives	$ (13)	$ (29)	$ (12)	$ (73)