Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information
Property-Liability net investment income	$ 575	$ 594
Net gains (losses) on investments and derivatives	14	(267)
Pension and other postretirement remeasurement gains (losses)	53	247
Amortization of purchased intangibles	(81)	(87)
Less: Net income (loss) attributable to noncontrolling interest	(1)	(10)
Net (loss) income applicable to common shareholders, Basic	(346)	634
Segment Reconciling Items
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	653	345
Net gains (losses) on investments and derivatives	14	(267)
Pension and other postretirement remeasurement gains (losses)	53	247
Amortization of purchased intangibles	(24)	(29)
Gain (loss) on disposition	9	(16)
Income tax benefit (expense) on reconciling items	92	(148)
Allstate Protection \| Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(998)	282
Run-off Property-Liability \| Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(3)	(2)
Property-Liability
Segment Reporting Information
Property-Liability net investment income	509	558
Net gains (losses) on investments and derivatives	12	(203)
Less: Net income (loss) attributable to noncontrolling interest	(1)	(10)
Property-Liability \| Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(1,001)	280
Property-Liability \| Segment Reconciling Items
Segment Reporting Information
Property-Liability net investment income	509	558
Protection Services \| Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	34	53
Allstate Health and Benefits
Segment Reporting Information
Property-Liability net investment income	19	17
Net gains (losses) on investments and derivatives	2	(7)
Allstate Health and Benefits \| Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	56	57
Corporate and Other \| Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	(89)	(111)
Property-Liability net investment income	31	10
Net gains (losses) on investments and derivatives	$ 1	$ (44)

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