Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information
Adjusted net income	$ 970	$ 234	$ 1,701	$ (267)
Net investment income	974	220	1,682	466
Realized capital gains (losses)	287	440	713	278
Pension and other postretirement remeasurement gains (losses)	134	(73)	444	(391)
Business combination expense and amortization of purchased intangibles	(34)	(26)	(90)	(53)
Business combination fair value adjustment	6	0	6	0
Income tax expense	(354)	(285)	(976)	(481)
Income (loss) from discontinued operations	493	167	(3,670)	(203)
Income tax (expense) benefit from discontinued operations	(297)	(23)	73	59
Income (loss) from discontinued operations, net of tax	196	144	(3,597)	(144)
Less: Net income attributable to noncontrolling interest	6	0	0	0
Net income applicable to common shareholders	1,595	1,224	187	1,737
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	431	905	2,091	2,256
Run-off Property-Liability
Segment Reporting Information
Underwriting income (loss)	(2)	(3)	(5)	(6)
Protection Services
Segment Reporting Information
Adjusted net income	56	38	105	75
Allstate Health and Benefits
Segment Reporting Information
Adjusted net income	62	5	127	29
Net investment income	19	20	38	40
Realized capital gains (losses)	4	11	6	(3)
Corporate and Other
Segment Reporting Information
Adjusted net income	(112)	(99)	(235)	(206)
Property-Liability
Segment Reporting Information
Net investment income	931	178	1,604	380
Realized capital gains (losses)	$ 265	$ 382	$ 669	$ 279

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