Reporting Segments - Reportable segments financial performance (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information
Net investment income									$ 3,401	$ 3,042	$ 3,156
Goodwill impairment	$ (125)								(125)	0	0
Net income applicable to common shareholders	$ 1,220	$ 637	$ 550	$ 666	$ 811	$ 491	$ 242	$ 217	3,073	1,761	2,055
Allstate Protection
Segment Reporting Information
Underwriting income									2,111	1,327	1,621
Property-Liability
Segment Reporting Information
Underwriting income									2,012	1,220	1,566
Net investment income									1,478	1,253	1,226
Income tax expense on operations									(1,119)	(812)	(922)
Realized capital gains and losses, after-tax									272	0	(154)
Gain (loss) on disposition of operations, after-tax									9	0	0
Change in accounting for investments in qualified affordable housing projects									0	0	(28)
Tax Legislation expense									(65)	0	0
Net income applicable to common shareholders									2,587	1,661	1,688
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income									(99)	(107)	(55)
Service Businesses
Segment Reporting Information
Net investment income									16	13	11
Tax Legislation expense									134	0	0
Net income applicable to common shareholders									15	3	2
Adjusted net (loss) income									(59)	3	2
Amortization of purchased intangible assets, after-tax									(60)	0	0
Allstate Life
Segment Reporting Information
Net investment income									489	482	490
Realized capital gains and losses, after-tax									2	(24)	1
Gain (loss) on disposition of operations, after-tax									0	0	(1)
Change in accounting for investments in qualified affordable housing projects									0	0	(6)
Tax Legislation expense									332	0	0
Net income applicable to common shareholders									577	219	229
Adjusted net (loss) income									253	247	239
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax									(10)	(4)	(4)
Allstate Benefits
Segment Reporting Information
Net investment income									72	71	71
Realized capital gains and losses, after-tax									0	(4)	0
Tax Legislation expense									51	0	0
Net income applicable to common shareholders									146	96	104
Adjusted net (loss) income									95	100	104
Allstate Annuities
Segment Reporting Information
Net investment income									1,305	1,181	1,323
Realized capital gains and losses, after-tax									28	(26)	172
Gain (loss) on disposition of operations, after-tax									4	3	3
Change in accounting for investments in qualified affordable housing projects									0	0	(11)
Tax Legislation expense									182	0	0
Net income applicable to common shareholders									418	76	330
Adjusted net (loss) income									204	101	166
Valuation changes on embedded derivatives that are not hedged, after-tax									0	(2)	(1)
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax									0	0	1
Corporate and Other
Segment Reporting Information
Net investment income									41	42	35
Realized capital gains and losses, after-tax									(4)	(2)	0
Goodwill impairment									(125)	0	0
Business combination expenses, after-tax									(14)	0	0
Tax Legislation expense									(128)	0	0
Net income applicable to common shareholders									(670)	(294)	(298)
Adjusted net (loss) income									$ (399)	$ (292)	$ (298)