Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information
Deferred policy acquisition costs	$ 4,191	$ 3,954	$ 3,861
Reserves for claims and claims expense, contract benefits and contractholder funds	58,308	57,749	57,411
Unearned premiums	13,473	12,583	12,202
Premium revenue and contract charges	34,678	33,582	32,467
Net investment income	3,401	3,042	3,156
Claims and claims expense, contract benefits and interest credited to contractholders	24,542	24,804	23,598
Amortization of deferred policy acquisition costs	4,784	4,550	4,364
Other operating costs and expenses	5,227	4,431	4,412
Premiums written (excluding life)	33,661	32,455	31,648
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	(110)	(105)	(42)
Net investment income	0	0	0
Claims and claims expense, contract benefits and interest credited to contractholders	(6)	(5)	(14)
Amortization of deferred policy acquisition costs	0	0	0
Other operating costs and expenses	(104)	(100)	(28)
Premiums written (excluding life)	0	0	0
Allstate Protection | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,510	1,432	1,410
Reserves for claims and claims expense, contract benefits and contractholder funds	24,336	23,263	21,777
Unearned premiums	11,409	11,160	10,979
Premium revenue and contract charges	31,433	30,727	29,748
Claims and claims expense, contract benefits and interest credited to contractholders	21,470	21,863	20,718
Amortization of deferred policy acquisition costs	4,205	4,053	3,933
Other operating costs and expenses	3,647	3,484	3,476
Premiums written (excluding life)	31,648	30,888	30,115
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	1,893	1,953	2,062
Unearned premiums	0	0	0
Premium revenue and contract charges	0	0	0
Claims and claims expense, contract benefits and interest credited to contractholders	96	105	53
Amortization of deferred policy acquisition costs	0	0	0
Other operating costs and expenses	3	2	2
Premiums written (excluding life)	0	3	0
Property-Liability | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,510	1,432	1,410
Reserves for claims and claims expense, contract benefits and contractholder funds	26,229	25,216	23,839
Unearned premiums	11,409	11,160	10,979
Premium revenue and contract charges	31,433	30,727	29,748
Net investment income	1,478	1,253	1,226
Claims and claims expense, contract benefits and interest credited to contractholders	21,566	21,968	20,771
Amortization of deferred policy acquisition costs	4,205	4,053	3,933
Other operating costs and expenses	3,650	3,486	3,478
Premiums written (excluding life)	31,648	30,891	30,115
Service Businesses | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	954	756	619
Reserves for claims and claims expense, contract benefits and contractholder funds	96	34	30
Unearned premiums	2,052	1,411	1,210
Premium revenue and contract charges	977	685	603
Net investment income	16	13	11
Claims and claims expense, contract benefits and interest credited to contractholders	369	258	277
Amortization of deferred policy acquisition costs	296	214	169
Other operating costs and expenses	506	223	164
Premiums written (excluding life)	1,094	709	756
Allstate Life | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	1,152	1,200	1,271
Reserves for claims and claims expense, contract benefits and contractholder funds	10,244	10,042	9,895
Unearned premiums	4	4	4
Premium revenue and contract charges	1,280	1,250	1,223
Net investment income	489	482	490
Claims and claims expense, contract benefits and interest credited to contractholders	1,047	1,027	1,031
Amortization of deferred policy acquisition costs	134	131	133
Other operating costs and expenses	240	226	213
Premiums written (excluding life)	0	0	0
Allstate Benefits | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	541	526	514
Reserves for claims and claims expense, contract benefits and contractholder funds	1,869	1,821	1,760
Unearned premiums	8	8	9
Premium revenue and contract charges	1,084	1,011	921
Net investment income	72	71	71
Claims and claims expense, contract benefits and interest credited to contractholders	599	545	488
Amortization of deferred policy acquisition costs	142	145	124
Other operating costs and expenses	269	240	222
Premiums written (excluding life)	919	855	777
Allstate Annuities | Operating Segments
Supplementary Insurance Information
Deferred policy acquisition costs	34	40	47
Reserves for claims and claims expense, contract benefits and contractholder funds	19,870	20,636	21,887
Unearned premiums	0	0	0
Premium revenue and contract charges	14	14	14
Net investment income	1,305	1,181	1,323
Claims and claims expense, contract benefits and interest credited to contractholders	967	1,011	1,045
Amortization of deferred policy acquisition costs	7	7	5
Other operating costs and expenses	35	32	37
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	41	42	35
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	631	324	326
Premiums written (excluding life)	$ 0	$ 0	$ 0