Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income from continuing operations	$ 2,406	$ 837
Less: Net loss attributable to noncontrolling interest	(6)	0
Net income from continuing operations attributable to Allstate	2,412	837
Less: Preferred stock dividends	27	36
Net income from continuing operations applicable to common shareholders	2,385	801
Loss from discontinued operations, net of tax	(3,793)	(288)
Net (loss) income applicable to common shareholders	$ (1,408)	$ 513
Denominator:
Weighted average common shares - Basic (in shares)	302.5	317.4
Effect of dilutive potential common shares:
Stock options (in shares)	2.5	3.3
Restricted stock units (non-participating) and performance stock awards (in shares)	1.4	1.7
Weighted average common and dilutive potential common shares outstanding (in shares)	306.4	322.4
Basic
Earnings per common share - Continuing operations basic (in dollars per share)	$ 7.88	$ 2.52
Earnings per common share - Discontinued operations basic (in dollars per share)	(12.53)	(0.90)
Earnings per common share - Basic (in dollars per share)	(4.65)	1.62
Diluted
Earnings per common share - Continuing operations diluted (in dollars per share)	7.78	2.48
Earnings per common share - Discontinued operations diluted (in dollars per share)	(12.38)	(0.89)
Earnings per common share - Diluted (in dollars per share)	$ (4.60)	$ 1.59
Anti-dilutive options excluded from diluted earnings per common share (in shares)	2.2	0.8