Earnings per Common Share - Computation earnings per common share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net income from continuing operations	$ 1,435	$ 1,106	$ 3,841	$ 1,943
Less: Net income attributable to noncontrolling interest	6	0	0	0
Net income from continuing operations attributable to Allstate	1,429	1,106	3,841	1,943
Less: Preferred stock dividends	30	26	57	62
Net income from continuing operations applicable to common shareholders	1,399	1,080	3,784	1,881
Income (loss) from discontinued operations, net of tax	196	144	(3,597)	(144)
Net income applicable to common shareholders	$ 1,595	$ 1,224	$ 187	$ 1,737
Denominator:
Weighted average common shares - Basic (in shares)	298.8	313.7	300.6	315.6
Effect of dilutive potential common shares:
Stock options (in shares)	3.2	2.1	2.8	2.7
Restricted stock units (non-participating) and performance stock awards (in shares)	1.3	1.2	1.5	1.5
Weighted average common and dilutive potential common shares outstanding (in shares)	303.3	317.0	304.9	319.8
Basic
Earnings per common share - Continuing operations basic (in dollars per share)	$ 4.68	$ 3.44	$ 12.59	$ 5.96
Earnings per common share - Discontinued operations basic (in dollars per share)	0.66	0.46	(11.97)	(0.46)
Earnings per common share - Basic (in dollars per share)	5.34	3.90	0.62	5.50
Diluted
Earnings per common share - Continuing operations diluted (in dollars per share)	4.61	3.41	12.41	5.88
Earnings per common share - Discontinued operations diluted (in dollars per share)	0.65	0.45	(11.80)	(0.45)
Earnings per common share - Diluted (in dollars per share)	$ 5.26	$ 3.86	$ 0.61	$ 5.43
Anti-dilutive options excluded from diluted earnings per common share (in shares)	0.8	3.2	2.0	1.4