Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information
Property-Liability net investment income	$ 690	$ 764	$ 1,846	$ 2,446
Net gains (losses) on investments and derivatives	(167)	105	(1,167)	818
Pension and other postretirement remeasurement gains (losses)	(79)	(40)	(91)	404
Income (loss) from discontinued operations	0	235	0	(3,435)
Income tax benefit from discontinued operations	0	90	0	163
Income (loss) from discontinued operations, net of tax	0	325	0	(3,272)
Less: Net income (loss) attributable to noncontrolling interest	(15)	(7)	(34)	(7)
Net (loss) income applicable to common shareholders	(694)	508	(1,106)	695
Segment Reconciling Items
Segment Reporting Information
Adjusted net income (loss)	598	727	754	2,428
Net gains (losses) on investments and derivatives	(167)	105	(1,167)	818
Pension and other postretirement remeasurement gains (losses)	(79)	(40)	(91)	404
Business combination expenses and amortization of purchased intangibles	(29)	(34)	(86)	(124)
Business combination fair value adjustment	0	0	0	6
Gain (loss) on disposition of operations	(5)	0	6	0
Income tax benefit (expense) on reconciling items	246	(14)	396	(990)
Allstate Protection \| Operating Segments
Segment Reporting Information
Underwriting income (loss)	(1,170)	(421)	(1,749)	1,670
Run-off Property-Liability \| Operating Segments
Segment Reporting Information
Underwriting income (loss)	(122)	(113)	(127)	(118)
Property-Liability
Segment Reporting Information
Property-Liability net investment income	632	710	1,696	2,314
Net gains (losses) on investments and derivatives	(123)	94	(988)	763
Less: Net income (loss) attributable to noncontrolling interest	(15)	(7)	(35)	(7)
Property-Liability \| Operating Segments
Segment Reporting Information
Underwriting income (loss)	(1,292)	(534)	(1,876)	1,552
Property-Liability \| Segment Reconciling Items
Segment Reporting Information
Property-Liability net investment income	632	710	1,696	2,314
Protection Services \| Operating Segments
Segment Reporting Information
Adjusted net income (loss)	35	45	131	150
Allstate Health and Benefits
Segment Reporting Information
Property-Liability net investment income	17	18	50	56
Net gains (losses) on investments and derivatives	(6)	(1)	(25)	5
Allstate Health and Benefits \| Operating Segments
Segment Reporting Information
Adjusted net income (loss)	54	33	172	160
Corporate and Other \| Operating Segments
Segment Reporting Information
Adjusted net income (loss)	(104)	(95)	(322)	(330)
Property-Liability net investment income	28	26	66	44
Net gains (losses) on investments and derivatives	$ (25)	$ 8	$ (98)	$ 30

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