Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Numerator:
Net income from continuing operations	$ 646	$ 2,406
Less: Net loss attributable to noncontrolling interest	(10)	(6)
Net income from continuing operations attributable to Allstate	656	2,412
Less: Preferred stock dividends	26	27
Net income from continuing operations applicable to common shareholders	630	2,385
Income (loss) from discontinued operations, net of tax	0	(3,793)
Net income (loss) applicable to common shareholders	$ 630	$ (1,408)
Denominator:
Weighted average common shares - Basic (in shares)	278.1	302.5
Effect of dilutive potential common shares:
Stock options (in shares)	2.6	2.5
Restricted stock units (non-participating) and performance stock awards (in shares)	1.1	1.4
Weighted average common and dilutive potential common shares outstanding (in shares)	281.8	306.4
Basic
Earnings per common share - Continuing operations basic (in dollars per share)	$ 2.27	$ 7.88
Earnings per common share - Discontinued operations basic (in dollars per share)	0	(12.53)
Earnings per common share - Basic (in dollars per share)	2.27	(4.65)
Diluted
Earnings per common share - Continuing operations diluted (in dollars per share)	2.24	7.78
Earnings per common share - Discontinued operations diluted (in dollars per share)	0	(12.38)
Earnings per common share - Diluted (in dollars per share)	$ 2.24	$ (4.60)
Anti-dilutive options excluded from diluted earnings per common share (in shares)	1.2	2.2