Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information
Net investment income	$ 648	$ 786
Net income applicable to common shareholders	1,261	977
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	703	1,008
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(3)	(3)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	700	1,005
Net investment income	291	337
Income tax expense on operations	(202)	(277)
Realized capital gains and losses, after-tax	393	(75)
Net income applicable to common shareholders	1,182	990
Service Businesses
Segment Reporting Information
Adjusted net income (loss)	11
Realized capital gains and losses, after-tax	7	(3)
Amortization of purchased intangible assets, after-tax	(24)	(16)
Net income applicable to common shareholders	(6)	(22)
Allstate Life
Segment Reporting Information
Adjusted net income (loss)	73
Realized capital gains and losses, after-tax	(4)	(2)
DAC and DSI amortization related to realized capital gains and losses, after-tax	(2)	(2)
Net income applicable to common shareholders	67	67
Allstate Benefits
Segment Reporting Information
Adjusted net income (loss)	31
Realized capital gains and losses, after-tax	3	(2)
Net income applicable to common shareholders	34	27
Allstate Annuities
Segment Reporting Information
Adjusted net income (loss)	(25)
Realized capital gains and losses, after-tax	124	(23)
Valuation changes on embedded derivatives not hedged, after-tax	(3)	4
Gain on disposition of operations, after-tax	1	1
Net income applicable to common shareholders	97	17
Corporate and Other
Segment Reporting Information
Adjusted net income (loss)	(103)
Realized capital gains and losses, after-tax	1	(1)
Pension and other postretirement remeasurement gains and losses, after-tax	(11)	(11)
Net income applicable to common shareholders	$ (113)	$ (102)