Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Numerator:
Net income (loss)	$ 347	$ (1,375)	$ 1,545	$ (1,696)
Less: Net income (loss) attributable to noncontrolling interest	16	(23)	(4)	(24)
Net income (loss) attributable to Allstate	331	(1,352)	1,549	(1,672)
Less: Preferred stock dividends	30	37	59	63
Net income (loss) applicable to common shareholders, basic	301	(1,389)	1,490	(1,735)
Net income (loss) applicable to common shareholders, diluted	$ 301	$ (1,389)	$ 1,490	$ (1,735)
Denominator:
Weighted average common shares outstanding (in shares)	264.1	262.6	263.8	263.1
Effect of dilutive potential common shares
Stock options (in shares)	2.5	0.0	2.5	0.0
Restricted stock units (non-participating) and performance stock awards (in shares)	0.5	0.0	0.5	0.0
Weighted average common and dilutive potential common shares outstanding (in shares)	267.1	262.6	266.8	263.1
Earnings per common share:
Earnings per common share - Basic (in dollars per share)	$ 1.14	$ (5.29)	$ 5.65	$ (6.59)
Earnings per common share - Diluted (in dollars per share)	$ 1.13	$ (5.29)	$ 5.58	$ (6.59)
Anti-dilutive options excluded from diluted earnings per common share (in shares)	0.6	3.2	0.5	2.9
Weighted average dilutive potential common shares excluded due to net loss applicable to common shareholders (in shares)	0.0	1.7	0.0	2.1