Summary of Significant Accounting Policies - Schedule of Consolidated Statements of Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues
Accident and health insurance premiums and contract charges									$ 52,516	$ 47,736	$ 44,052
Total revenues	$ 14,832	$ 14,497	$ 13,979	$ 13,786	$ 13,648	$ 13,208	$ 12,219	$ 12,336	57,094	51,411	50,601
Costs and expenses
Accident, health and other policy benefits									1,071	1,042	1,060
Amortization of deferred policy acquisition costs									7,278	6,634	6,236
Total costs and expenses									57,442	53,241	44,135
(Loss) income from operations before income tax expense									(348)	(1,830)	6,466
Income tax (benefit) expense									(135)	(488)	1,292
Net (loss) income from continuing operations									(213)	(1,342)	5,174
Net (loss) income									(213)	(1,342)	1,581
Net (loss) income attributable to Allstate									(188)	(1,289)	1,614
Net (loss) income applicable to common shareholders, basic	1,460	(41)	(1,389)	(346)	(303)	(685)	(1,040)	634	(316)	(1,394)	1,500
Net (loss) income applicable to common shareholders, diluted	$ 1,460	$ (41)	$ (1,389)	$ (346)	$ (303)	$ (685)	$ (1,040)	$ 634	$ (316)	$ (1,394)	$ 1,500
Earnings per common share:
Net income (loss) applicable to common shareholders per common share - Basic (in dollars per share)	$ 5.57	$ (0.16)	$ (5.29)	$ (1.31)	$ (1.15)	$ (2.55)	$ (3.80)	$ 2.28	$ (1.20)	$ (5.14)	$ 5.09
Net income (loss) applicable to common shareholders per common share - Diluted (in dollars per share)	$ 5.52	$ (0.16)	$ (5.29)	$ (1.31)	$ (1.15)	$ (2.55)	$ (3.80)	$ 2.25	(1.20)	(5.14)	5.01
Net income from continuing operations applicable to common shareholders per common share - Basic (in dollars per share)									(1.20)	(5.14)	17.28
Net income from continuing operations applicable to common shareholders per common share - Diluted (in dollars per share)									$ (1.20)	$ (5.14)	$ 17.03
As reported
Revenues
Total revenues										$ 51,412	$ 50,588
Costs and expenses
Accident, health and other policy benefits										1,061	1,049
Amortization of deferred policy acquisition costs										6,644	6,252
Total costs and expenses										53,270	44,140
(Loss) income from operations before income tax expense										(1,858)	6,448
Income tax (benefit) expense										(494)	1,289
Net (loss) income from continuing operations											5,159
Net (loss) income										(1,364)	1,566
Net (loss) income attributable to Allstate										(1,311)	1,599
Net (loss) income applicable to common shareholders, basic										(1,416)	1,485
Net (loss) income applicable to common shareholders, diluted										$ (1,416)	$ 1,485
Earnings per common share:
Net income (loss) applicable to common shareholders per common share - Basic (in dollars per share)										$ (5.22)	$ 5.04
Net income (loss) applicable to common shareholders per common share - Diluted (in dollars per share)										$ (5.22)	4.96
Net income from continuing operations applicable to common shareholders per common share - Basic (in dollars per share)											17.23
Net income from continuing operations applicable to common shareholders per common share - Diluted (in dollars per share)											$ 16.98
Impact of change
Revenues
Total revenues										$ (1)	$ 13
Costs and expenses
Accident, health and other policy benefits										(19)	11
Amortization of deferred policy acquisition costs										(10)	(16)
Total costs and expenses										(29)	(5)
(Loss) income from operations before income tax expense										28	18
Income tax (benefit) expense										6	3
Net (loss) income from continuing operations											15
Net (loss) income										22	15
Net (loss) income attributable to Allstate										22	15
Net (loss) income applicable to common shareholders, basic										22	15
Net (loss) income applicable to common shareholders, diluted										$ 22	$ 15
Earnings per common share:
Net income (loss) applicable to common shareholders per common share - Basic (in dollars per share)										$ 0.08	$ 0.05
Net income (loss) applicable to common shareholders per common share - Diluted (in dollars per share)										$ 0.08	0.05
Net income from continuing operations applicable to common shareholders per common share - Basic (in dollars per share)											0.05
Net income from continuing operations applicable to common shareholders per common share - Diluted (in dollars per share)											$ 0.05
Accident and health insurance premiums and contract charges
Revenues
Accident and health insurance premiums and contract charges									$ 1,846	$ 1,832	$ 1,834
Accident and health insurance premiums and contract charges | As reported
Revenues
Accident and health insurance premiums and contract charges										1,833	1,821
Accident and health insurance premiums and contract charges | Impact of change
Revenues
Accident and health insurance premiums and contract charges										$ (1)	$ 13