Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 531	$ 1,153	$ 775	$ 2,952
Changes in:
Unrealized net capital gains and losses	(336)	142	(1,352)	857
Unrealized foreign currency translation adjustments	(21)	25	10	(5)
Unamortized pension and other postretirement prior service credit	(15)	38	(44)	31
Other comprehensive (loss) income, after-tax	(372)	205	(1,386)	883
Comprehensive income (loss)	159	1,358	(611)	3,835
Less: Comprehensive loss attributable to noncontrolling interest	(7)	0	(8)	0
Comprehensive income (loss) attributable to Allstate	$ 166	$ 1,358	$ (603)	$ 3,835