Reporting Segments - Reportable segments financial performance (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information
Net investment income									$ 3,240	$ 3,401	$ 3,042
Goodwill impairment					$ (125)				0	(125)	0
Net income applicable to common shareholders	$ (312)	$ 833	$ 637	$ 946	$ 1,220	$ 637	$ 550	$ 666	2,104	3,073	1,761
Allstate Protection
Segment Reporting Information
Underwriting income									2,187	2,111	1,327
Property-Liability
Segment Reporting Information
Underwriting income									2,097	2,012	1,220
Net investment income									1,464	1,478	1,253
Income tax expense on operations									(715)	(1,119)	(812)
Realized capital gains and losses, after-tax									(500)	272	0
Gain (loss) on disposition of operations, after-tax									0	9	0
Tax Legislation expense									(5)	(65)	0
Net income applicable to common shareholders									2,341	2,587	1,661
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income									(90)	(99)	(107)
Service Businesses
Segment Reporting Information
Net investment income									27	16	13
Realized capital gains and losses, after-tax									(9)	0	0
Tax Legislation expense									(4)	134	0
Net income applicable to common shareholders									(85)	15	3
Adjusted net income (loss)									2	(59)	3
Amortization of purchased intangible assets, after-tax									(74)	(60)	0
Allstate Life
Segment Reporting Information
Net investment income									505	489	482
Realized capital gains and losses, after-tax									(11)	2	(24)
Tax Legislation expense									(16)	332	0
Net income applicable to common shareholders									254	577	219
Adjusted net income (loss)									289	253	247
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax									(8)	(10)	(4)
Allstate Benefits
Segment Reporting Information
Net investment income									77	72	71
Realized capital gains and losses, after-tax									(7)	0	(4)
Tax Legislation expense									0	51	0
Net income applicable to common shareholders									113	146	96
Adjusted net income (loss)									119	95	100
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax									1	0	0
Allstate Annuities
Segment Reporting Information
Net investment income									1,096	1,305	1,181
Realized capital gains and losses, after-tax									(131)	28	(26)
Gain (loss) on disposition of operations, after-tax									4	4	3
Tax Legislation expense									69	182	0
Net income applicable to common shareholders									75	418	76
Adjusted net income (loss)									130	204	101
Valuation changes on embedded derivatives that are not hedged, after-tax									3	0	(2)
Corporate and Other
Segment Reporting Information
Net investment income									71	41	42
Realized capital gains and losses, after-tax									(30)	(4)	(2)
Tax Legislation expense									(15)	(128)	0
Goodwill impairment									0	(125)	0
Business combination expenses, after-tax									(7)	(14)	0
Net income applicable to common shareholders									(594)	(670)	(294)
Adjusted net income (loss)									$ (542)	$ (399)	$ (292)