Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information
Net investment income	$ 594	$ 708
Net gains (losses) on investments and derivatives	(267)	426
Pension and other postretirement remeasurement gains (losses)	247	310
Loss from discontinued operations	0	(4,163)
Income tax benefit from discontinued operations	0	370
Income (loss) from discontinued operations, net of tax	0	(3,793)
Less: Net loss attributable to noncontrolling interest	(10)	(6)
Net income (loss) applicable to common shareholders	630	(1,408)
Segment Reconciling Items
Segment Reporting Information
Adjusted net income	345	731
Net investment income	558	673
Net gains (losses) on investments and derivatives	(267)	426
Pension and other postretirement remeasurement gains (losses)	247	310
Business combination expense and amortization of purchased intangibles	(29)	(56)
Gain (loss) on disposition of operations	(16)	0
Income tax expense	(148)	(622)
Allstate Protection | Operating Segments
Segment Reporting Information
Underwriting income (loss)	282	1,660
Run-off Property-Liability | Operating Segments
Segment Reporting Information
Underwriting income (loss)	(2)	(3)
Property-Liability
Segment Reporting Information
Net investment income	558	673
Net gains (losses) on investments and derivatives	(203)	404
Property-Liability | Operating Segments
Segment Reporting Information
Underwriting income (loss)	280	1,657
Protection Services | Operating Segments
Segment Reporting Information
Adjusted net income	53	49
Allstate Health and Benefits
Segment Reporting Information
Net investment income	17	19
Net gains (losses) on investments and derivatives	(7)	2
Allstate Health and Benefits | Operating Segments
Segment Reporting Information
Adjusted net income	53	65
Corporate and Other | Operating Segments
Segment Reporting Information
Adjusted net income	(111)	(123)
Net investment income	10	6
Net gains (losses) on investments and derivatives	$ (44)	$ 10