Reportable Segments - Schedule of Reportable Segments Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information
Net investment income	$ 854	$ 764
Net gains (losses) on investments and derivatives	(349)	(164)
Pension and other postretirement remeasurement gains (losses)	(78)	2
Amortization of purchased intangibles	(59)	(69)
Less: Net income (loss) attributable to noncontrolling interest	1	(20)
Net income applicable to common shareholders, basic	566	1,189
Net income applicable to common shareholders, diluted	566	1,189
Operating segments
Segment Reporting Information
Amortization of purchased intangibles	(59)	(69)
Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	219	267
Net gains (losses) on investments and derivatives	(349)	(164)
Pension and other postretirement remeasurement gains (losses)	(78)	2
Amortization of purchased intangibles	(13)	(18)
Gain (loss) on disposition	0	4
Income tax (expense) benefit on Property-Liability and reconciling items	(124)	(259)
Property-Liability
Segment Reporting Information
Less: Net income (loss) attributable to noncontrolling interest	1	(20)
Property-Liability | Operating segments
Segment Reporting Information
Underwriting income (loss) by segment	360	898
Net investment income	783	702
Net gains (losses) on investments and derivatives	(329)	(162)
Amortization of purchased intangibles	(46)	(51)
Property-Liability | Operating segments | Allstate Protection
Segment Reporting Information
Underwriting income (loss) by segment	364	903
Property-Liability | Operating segments | Run-off Property-Liability
Segment Reporting Information
Underwriting income (loss) by segment	(4)	(5)
Property-Liability | Segment Reporting, Reconciling Item, Excluding Corporate Nonsegment
Segment Reporting Information
Net investment income	783	702
Protection Services | Operating segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	55	54
Net investment income	24	21
Net gains (losses) on investments and derivatives	(10)	(5)
Amortization of purchased intangibles	(9)	(11)
Allstate Health and Benefits | Operating segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	30	56
Net investment income	25	23
Net gains (losses) on investments and derivatives	(1)	2
Amortization of purchased intangibles	(4)	(7)
Corporate and Other | Operating segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	(97)	(106)
Net investment income	22	18
Net gains (losses) on investments and derivatives	$ (9)	$ 1