Summary of Significant Accounting Policies - Schedule of Consolidated Statements of Financial Position (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Assets
Deferred policy acquisition costs	$ 5,940	$ 5,442	$ 4,738	$ 3,776
Reinsurance and indemnification recoverables, net	8,809	9,619
Deferred income taxes	219	382
Other assets, net	6,051	5,904
Total assets	103,362	97,989
Liabilities
Reserve for future policy benefits	1,347	1,322
Contractholder funds	888	879	890	857
Unearned premiums	24,709	22,299
Total liabilities	85,732	80,626
Equity
Retained income	49,716	50,970
Accumulated other comprehensive income:
Unrealized net capital gains and losses	(604)	(2,255)
Discount rate for reserve for future policy benefits	(11)	(1)
Total accumulated other comprehensive loss	(700)	(2,392)
Total Allstate shareholders’ equity	17,770	17,488	24,944
Total equity	17,630	17,363	24,892	$ (298)
Total liabilities and equity	$ 103,362	97,989
As reported
Assets
Deferred policy acquisition costs		5,418
Reinsurance and indemnification recoverables, net		9,606
Deferred income taxes		386
Other assets, net		5,905
Total assets		97,957
Liabilities
Reserve for future policy benefits		1,273
Contractholder funds		897
Unearned premiums		22,311
Total liabilities		80,607
Equity
Retained income		50,954
Accumulated other comprehensive income:
Unrealized net capital gains and losses		(2,253)
Discount rate for reserve for future policy benefits		0
Total accumulated other comprehensive loss		(2,389)
Total Allstate shareholders’ equity		17,475	25,179
Total equity		17,350	25,127
Total liabilities and equity		97,957
Impact of change
Assets
Deferred policy acquisition costs		24
Reinsurance and indemnification recoverables, net		13
Deferred income taxes		(4)
Other assets, net		(1)
Total assets		32
Liabilities
Reserve for future policy benefits		49
Contractholder funds		(18)
Unearned premiums		(12)
Total liabilities		19
Equity
Retained income		16
Accumulated other comprehensive income:
Unrealized net capital gains and losses		(2)
Discount rate for reserve for future policy benefits		(1)
Total accumulated other comprehensive loss		(3)
Total Allstate shareholders’ equity		13	(235)
Total equity		13	$ (235)
Total liabilities and equity		$ 32