Reportable Segments - Schedule of Reportable Segments Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information
Allstate Protection and Run-off Property-Liability net investment income	$ 764	$ 575
Net gains (losses) on investments and derivatives	(164)	14
Pension and other postretirement remeasurement gains (losses)	2	53
Amortization of purchased intangibles	(69)	(81)
Less: Net loss attributable to noncontrolling interest	(20)	(1)
Net income (loss) applicable to common shareholders, basic	1,189	(346)
Net income (loss) applicable to common shareholders, diluted	1,189	(346)
Segment Reconciling Items
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	267	653
Net gains (losses) on investments and derivatives	(164)	14
Pension and other postretirement remeasurement gains (losses)	2	53
Amortization of purchased intangibles	(18)	(24)
Gain (loss) on disposition	4	9
Income tax (expense) benefit on reconciling items	(259)	92
Property-Liability
Segment Reporting Information
Less: Net loss attributable to noncontrolling interest	(20)	(1)
Property-Liability \| Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	898	(1,001)
Allstate Protection and Run-off Property-Liability net investment income	702	509
Net gains (losses) on investments and derivatives	(162)	12
Property-Liability \| Operating Segments \| Allstate Protection
Segment Reporting Information
Underwriting income (loss) by segment	903	(998)
Property-Liability \| Operating Segments \| Run-off Property-Liability
Segment Reporting Information
Underwriting income (loss) by segment	(5)	(3)
Property-Liability \| Segment Reconciling Items
Segment Reporting Information
Allstate Protection and Run-off Property-Liability net investment income	702	509
Protection Services \| Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	54	34
Allstate Protection and Run-off Property-Liability net investment income	21	16
Net gains (losses) on investments and derivatives	(5)	(1)
Allstate Health and Benefits \| Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	56	56
Allstate Protection and Run-off Property-Liability net investment income	23	19
Net gains (losses) on investments and derivatives	2	2
Corporate and Other \| Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	(106)	(89)
Allstate Protection and Run-off Property-Liability net investment income	18	31
Net gains (losses) on investments and derivatives	$ 1	$ 1

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