Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information
Deferred policy acquisition costs	$ 4,699	$ 4,784	$ 4,191
Reserves for claims and claims expense, contract benefits and contractholder funds	57,704	58,002	58,308
Unearned premiums	15,343	14,510	13,473
Premium revenue and contract charges	38,577	36,513	34,678
Net investment income	3,159	3,240	3,401
Claims and claims expense, contract benefits and interest credited to contractholders	26,655	25,405	24,460
Amortization of deferred policy acquisition costs	5,533	5,222	4,784
Other operating costs and expenses	6,404	6,566	5,634
Premiums written (excluding life)	37,942	35,966	33,661
Intersegment Eliminations (2)
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	(154)	(122)	(110)
Net investment income	0	0	0
Claims and claims expense, contract benefits and interest credited to contractholders	(9)	(7)	(6)
Amortization of deferred policy acquisition costs	0	0	0
Other operating costs and expenses	(145)	(115)	(104)
Premiums written (excluding life)	0	0	0
Allstate Protection | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,624	1,618	1,510
Reserves for claims and claims expense, contract benefits and contractholder funds	25,843	25,495	24,336
Unearned premiums	12,567	11,953	11,409
Premium revenue and contract charges	34,843	32,950	31,433
Claims and claims expense, contract benefits and interest credited to contractholders	23,517	22,348	21,388
Amortization of deferred policy acquisition costs	4,649	4,475	4,205
Other operating costs and expenses	4,506	4,522	4,239
Premiums written (excluding life)	35,419	33,555	31,648
Discontinued Lines and Coverages | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	0	0	0
Reserves for claims and claims expense, contract benefits and contractholder funds	1,818	1,864	1,893
Unearned premiums	0	0	0
Premium revenue and contract charges	0	0	0
Claims and claims expense, contract benefits and interest credited to contractholders	105	87	96
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-Liability | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,624	1,618	1,510
Reserves for claims and claims expense, contract benefits and contractholder funds	27,661	27,359	26,229
Unearned premiums	12,567	11,953	11,409
Premium revenue and contract charges	34,843	32,950	31,433
Net investment income	1,533	1,464	1,478
Claims and claims expense, contract benefits and interest credited to contractholders	23,622	22,435	21,484
Amortization of deferred policy acquisition costs	4,649	4,475	4,205
Other operating costs and expenses	4,509	4,525	4,242
Premiums written (excluding life)	35,419	33,555	31,648
Service Businesses | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,449	1,290	954
Reserves for claims and claims expense, contract benefits and contractholder funds	51	64	96
Unearned premiums	2,765	2,546	2,052
Premium revenue and contract charges	1,387	1,220	977
Net investment income	42	27	16
Claims and claims expense, contract benefits and interest credited to contractholders	363	350	369
Amortization of deferred policy acquisition costs	543	463	296
Other operating costs and expenses	838	603	565
Premiums written (excluding life)	1,535	1,431	1,094
Allstate Life | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,079	1,300	1,152
Reserves for claims and claims expense, contract benefits and contractholder funds	10,541	10,333	10,244
Unearned premiums	3	3	4
Premium revenue and contract charges	1,343	1,315	1,280
Net investment income	514	505	489
Claims and claims expense, contract benefits and interest credited to contractholders	1,154	1,094	1,047
Amortization of deferred policy acquisition costs	173	132	134
Other operating costs and expenses	356	364	344
Premiums written (excluding life)	0	0	0
Allstate Benefits | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	527	549	541
Reserves for claims and claims expense, contract benefits and contractholder funds	1,950	1,905	1,869
Unearned premiums	8	8	8
Premium revenue and contract charges	1,145	1,135	1,084
Net investment income	83	77	72
Claims and claims expense, contract benefits and interest credited to contractholders	635	630	599
Amortization of deferred policy acquisition costs	161	145	142
Other operating costs and expenses	285	278	261
Premiums written (excluding life)	988	980	919
Allstate Annuities | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	20	27	34
Reserves for claims and claims expense, contract benefits and contractholder funds	17,501	18,341	19,870
Unearned premiums	0	0	0
Premium revenue and contract charges	13	15	14
Net investment income	917	1,096	1,305
Claims and claims expense, contract benefits and interest credited to contractholders	890	903	967
Amortization of deferred policy acquisition costs	7	7	7
Other operating costs and expenses	30	31	34
Premiums written (excluding life)	0	0	0
Corporate and Other | Operating Segments
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	70	71	41
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	531	880	292
Premiums written (excluding life)	$ 0	$ 0	$ 0