Reportable Segments - Schedule of Reportable Segments Financial Performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information
Property-Liability net investment income	$ 689	$ 690	$ 1,874	$ 1,846
Net gains (losses) on investments and derivatives	(86)	(167)	(223)	(1,167)
Pension and other postretirement remeasurement gains (losses)	(149)	(79)	(56)	(91)
Amortization of purchased intangibles	(83)	(90)	(246)	(264)
Less: Net income (loss) attributable to noncontrolling interest	1	(15)	(23)	(34)
Net loss applicable to common shareholders, basic	(41)	(685)	(1,776)	(1,091)
Net loss applicable to common shareholders, diluted	(41)	(685)	(1,776)	(1,091)
Segment Reconciling Items
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	389	598	1,737	754
Net gains (losses) on investments and derivatives	(86)	(167)	(223)	(1,167)
Pension and other postretirement remeasurement gains (losses)	(149)	(79)	(56)	(91)
Amortization of purchased intangibles	(23)	(29)	(71)	(86)
Gain (loss) on disposition	(5)	(5)	(4)	6
Non-recurring costs	0	0	(90)	0
Income tax benefit on reconciling items	25	246	501	396
Property-Liability
Segment Reporting Information
Less: Net income (loss) attributable to noncontrolling interest	2	(15)	(22)	(35)
Property-Liability | Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(414)	(1,292)	(3,509)	(1,876)
Property-Liability net investment income	627	632	1,680	1,696
Net gains (losses) on investments and derivatives	(62)	(123)	(185)	(988)
Property-Liability | Segment Reconciling Items
Segment Reporting Information
Property-Liability net investment income	627	632	1,680	1,696
Allstate Protection | Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(331)	(1,170)	(3,421)	(1,749)
Run-off Property-Liability | Operating Segments
Segment Reporting Information
Underwriting income (loss) by segment	(83)	(122)	(88)	(127)
Protection Services | Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	27	35	102	131
Property-Liability net investment income	19	13	53	34
Net gains (losses) on investments and derivatives	(8)	(13)	(13)	(56)
Allstate Health and Benefits | Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	69	63	182	187
Property-Liability net investment income	20	17	60	50
Net gains (losses) on investments and derivatives	(2)	(6)	1	(25)
Corporate and Other | Operating Segments
Segment Reporting Information
Adjusted net income (loss) by segment, after-tax	(110)	(104)	(310)	(322)
Property-Liability net investment income	23	28	81	66
Net gains (losses) on investments and derivatives	$ (14)	$ (25)	$ (26)	$ (98)