Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information
Net investment income	$ 786	$ 748
Net income applicable to common shareholders	946	666
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	962	550
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(3)	(2)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	959	548
Net investment income	337	308
Income tax expense on operations	(268)	(268)
Realized capital gains and losses, after-tax	(75)	89
Net income applicable to common shareholders	953	677
Service Businesses
Segment Reporting Information
Realized capital gains and losses, after-tax	(3)	0
Net income applicable to common shareholders	(24)	(25)
Adjusted net loss	(5)	(10)
Amortization of purchased intangible assets, after-tax	(16)	(15)
Allstate Life
Segment Reporting Information
Realized capital gains and losses, after-tax	(2)	1
Net income applicable to common shareholders	65	57
Adjusted net loss	69	59
DAC and DSI amortization related to realized capital gains and losses, after-tax	(2)	(3)
Allstate Benefits
Segment Reporting Information
Realized capital gains and losses, after-tax	(2)	0
Net income applicable to common shareholders	26	22
Adjusted net loss	28	22
Allstate Annuities
Segment Reporting Information
Realized capital gains and losses, after-tax	(23)	(2)
Net income applicable to common shareholders	17	29
Adjusted net loss	35	29
Valuation changes on embedded derivatives not hedged, after-tax	4	0
Gain on disposition of operations, after-tax	1	2
Corporate and Other
Segment Reporting Information
Realized capital gains and losses, after-tax	(1)	0
Net income applicable to common shareholders	(91)	(94)
Adjusted net loss	(90)	(81)
Business combination expenses, after-tax	$ 0	$ (13)