Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information
Adjusted net income	$ 731	$ (501)
Net investment income	708	246
Realized capital gains (losses)	426	(162)
Pension and other postretirement remeasurement (gains) losses	(310)	318
Business combination expenses and amortization of purchased intangibles	(56)	(27)
Income tax expense	(622)	(196)
Loss from discontinued operations	(4,163)	(370)
Income tax benefit from discontinued operations	(370)	(82)
Loss from discontinued operations, net of tax	(3,793)	(288)
Less: Net loss attributable to noncontrolling interest	(6)	0
Net (loss) income applicable to common shareholders	(1,408)	513
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	1,660	1,351
Run-off Property-Liability
Segment Reporting Information
Underwriting income (loss)	(3)	(3)
Protection Services
Segment Reporting Information
Adjusted net income	49	37
Allstate Health and Benefits
Segment Reporting Information
Adjusted net income	65	24
Net investment income	19	20
Realized capital gains (losses)	2	(14)
Corporate and Other
Segment Reporting Information
Adjusted net income	(123)	(107)
Property-Liability
Segment Reporting Information
Net investment income	673	202
Realized capital gains (losses)	$ 404	$ (103)