Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (1,387)	$ 549
Changes in:
Unrealized net capital gains and losses	(1,500)	(1,357)
Unrealized foreign currency translation adjustments	34	(39)
Unamortized pension and other postretirement prior service credit	(15)	4
Other comprehensive loss, after-tax	(1,481)	(1,392)
Comprehensive loss	(2,868)	(843)
Less: Comprehensive loss attributable to noncontrolling interest	(6)	0
Comprehensive loss applicable to Allstate	$ (2,862)	$ (843)