Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplementary Insurance Information
Deferred policy acquisition costs	$ 4,700	$ 4,699	$ 4,784
Reserves for claims and claims expense, contract benefits and contractholder funds	57,591	57,704	58,002
Unearned premiums	15,949	15,343	14,510
Premium revenue and contract charges	39,517	38,577	36,513
Net investment income	2,853	3,159	3,240
Claims and claims expense, contract benefits and interest credited to contractholders	24,882	26,655	25,405
Amortization of deferred policy acquisition costs	5,630	5,533	5,222
Other operating costs and expenses	7,324	6,404	6,566
Premiums written (excluding life)	38,584	37,942	35,966
Intersegment Eliminations
Supplementary Insurance Information
Deferred policy acquisition costs	0	0	0
Reserves for claims and claims expense, contract benefits and contractholder funds	0	0	0
Unearned premiums	0	0	0
Premium revenue and contract charges	(147)	(154)	(122)
Net investment income	0	0	0
Claims and claims expense, contract benefits and interest credited to contractholders	(11)	(9)	(7)
Amortization of deferred policy acquisition costs	0	0	0
Other operating costs and expenses	(136)	(145)	(115)
Premiums written (excluding life)	0	0	0
Allstate Protection | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	1,608	1,624	1,618
Reserves for claims and claims expense, contract benefits and contractholder funds	25,679	25,843	25,495
Unearned premiums	12,772	12,567	11,953
Premium revenue and contract charges	35,580	34,843	32,950
Claims and claims expense, contract benefits and interest credited to contractholders	21,485	23,517	22,348
Amortization of deferred policy acquisition costs	4,642	4,649	4,475
Other operating costs and expenses	5,623	4,506	4,522
Premiums written (excluding life)	35,768	35,419	33,555
Discontinued Lines and Coverages | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	0	0	0
Reserves for claims and claims expense, contract benefits and contractholder funds	1,888	1,818	1,864
Unearned premiums	0	0	0
Premium revenue and contract charges	0	0	0
Claims and claims expense, contract benefits and interest credited to contractholders	141	105	87
Amortization of deferred policy acquisition costs	0	0	0
Other operating costs and expenses	3	3	3
Premiums written (excluding life)	0	0	0
Property and casualty | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	1,608	1,624	1,618
Reserves for claims and claims expense, contract benefits and contractholder funds	27,567	27,661	27,359
Unearned premiums	12,772	12,567	11,953
Premium revenue and contract charges	35,580	34,843	32,950
Net investment income	1,421	1,533	1,464
Claims and claims expense, contract benefits and interest credited to contractholders	21,626	23,622	22,435
Amortization of deferred policy acquisition costs	4,642	4,649	4,475
Other operating costs and expenses	5,626	4,509	4,525
Premiums written (excluding life)	35,768	35,419	33,555
Protection Services | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	1,696	1,449	1,290
Reserves for claims and claims expense, contract benefits and contractholder funds	43	51	64
Unearned premiums	3,167	2,765	2,546
Premium revenue and contract charges	1,640	1,387	1,220
Net investment income	44	42	27
Claims and claims expense, contract benefits and interest credited to contractholders	386	363	350
Amortization of deferred policy acquisition costs	658	543	463
Other operating costs and expenses	760	838	603
Premiums written (excluding life)	1,890	1,535	1,431
Allstate Life | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	909	1,079	1,300
Reserves for claims and claims expense, contract benefits and contractholder funds	10,768	10,541	10,333
Unearned premiums	3	3	3
Premium revenue and contract charges	1,340	1,343	1,315
Net investment income	502	514	505
Claims and claims expense, contract benefits and interest credited to contractholders	1,293	1,154	1,094
Amortization of deferred policy acquisition costs	149	173	132
Other operating costs and expenses	335	356	364
Premiums written (excluding life)	0	0	0
Allstate Benefits | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	470	527	549
Reserves for claims and claims expense, contract benefits and contractholder funds	1,885	1,950	1,905
Unearned premiums	7	8	8
Premium revenue and contract charges	1,094	1,145	1,135
Net investment income	78	83	77
Claims and claims expense, contract benefits and interest credited to contractholders	549	635	630
Amortization of deferred policy acquisition costs	177	161	145
Other operating costs and expenses	323	285	278
Premiums written (excluding life)	926	988	980
Allstate Annuities | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	17	20	27
Reserves for claims and claims expense, contract benefits and contractholder funds	17,328	17,501	18,341
Unearned premiums	0	0	0
Premium revenue and contract charges	10	13	15
Net investment income	761	917	1,096
Claims and claims expense, contract benefits and interest credited to contractholders	1,039	890	903
Amortization of deferred policy acquisition costs	4	7	7
Other operating costs and expenses	27	30	31
Premiums written (excluding life)	0	0	0
Corporate and Other | Other revenue
Supplementary Insurance Information
Deferred policy acquisition costs	0	0	0
Reserves for claims and claims expense, contract benefits and contractholder funds	0	0	0
Unearned premiums	0	0	0
Premium revenue and contract charges	0	0	0
Net investment income	47	70	71
Claims and claims expense, contract benefits and interest credited to contractholders	0	0	0
Amortization of deferred policy acquisition costs	0	0	0
Other operating costs and expenses	389	531	880
Premiums written (excluding life)	$ 0	$ 0	$ 0