Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information
Adjusted net income	$ 727	$ 475	$ 2,428	$ 208
Net investment income	764	464	2,446	930
Realized capital gains (losses)	105	319	818	597
Pension and other postretirement remeasurement gains (losses)	(40)	71	404	(320)
Curtailment gains (losses)	0	8	0	8
Business combination expense and amortization of purchased intangibles	(34)	(28)	(124)	(81)
Business combination fair value adjustment	0	0	6	0
Income tax expense	(14)	(317)	(990)	(798)
Income (loss) from discontinued operations	235	(86)	(3,435)	(289)
Income tax benefit from discontinued operations	90	23	163	82
Income (loss) from discontinued operations, net of tax	325	(63)	(3,272)	(207)
Less: Net loss attributable to noncontrolling interest	(7)	0	(7)	0
Net income applicable to common shareholders	508	1,126	695	2,863
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	(421)	887	1,670	3,143
Run-off Property-Liability
Segment Reporting Information
Underwriting income (loss)	(113)	(135)	(118)	(141)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	(534)	752	1,552	3,002
Net investment income	710	422	2,314	802
Realized capital gains (losses)	94	292	763	571
Protection Services
Segment Reporting Information
Adjusted net income	45	40	150	115
Allstate Health and Benefits
Segment Reporting Information
Adjusted net income	33	33	160	62
Net investment income	18	18	56	58
Realized capital gains (losses)	(1)	3	5	0
Corporate and Other
Segment Reporting Information
Adjusted net income	$ (95)	$ (111)	$ (330)	$ (317)