Summary of Significant Accounting Policies - Schedule of Consolidated Statements of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net (loss) income	$ (213)	$ (1,342)	$ 1,581
Changes in:
Unrealized net capital gains and losses	1,651	(2,853)	(2,583)
Discount rate for reserve for future policy benefits	(10)	228	49
Other comprehensive income (loss), after-tax	1,692	(2,818)	(2,601)
Comprehensive income (loss)	1,479	(4,160)	(1,020)
Comprehensive income (loss) attributable to Allstate	$ 1,494	(4,087)	(984)
As reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net (loss) income		(1,364)	1,566
Changes in:
Unrealized net capital gains and losses		(2,851)	(2,582)
Discount rate for reserve for future policy benefits		0	0
Other comprehensive income (loss), after-tax		(3,044)	(2,649)
Comprehensive income (loss)		(4,408)	(1,083)
Comprehensive income (loss) attributable to Allstate		(4,335)	(1,047)
Impact of change
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net (loss) income		22	15
Changes in:
Unrealized net capital gains and losses		(2)	(1)
Discount rate for reserve for future policy benefits		228	49
Other comprehensive income (loss), after-tax		226	48
Comprehensive income (loss)		248	63
Comprehensive income (loss) attributable to Allstate		$ 248	$ 63