Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information
Net investment income	$ 409		$ 942	$ 830	$ 1,590
Net income applicable to common shareholders	1,224		821	1,737	2,082
Shelter-in-Place Payback expense	738	$ 210	0	948	0
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	907		370	2,255	1,073
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(3)		(3)	(6)	(6)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	904		367	2,249	1,067
Net investment income	178		471	380	762
Income tax expense on operations	(209)		(179)	(512)	(381)
Realized capital gains (losses), after-tax	299		204	217	597
Net income applicable to common shareholders	1,172		863	2,334	2,045
Service Businesses
Segment Reporting Information
Net investment income	11		10	21	19
Realized capital gains (losses), after-tax	15		6	(4)	13
Adjusted net income	38		16	75	27
Amortization of purchased intangibles, after-tax	(21)		(25)	(42)	(49)
Impairment of purchased intangibles, after-tax	0		(43)	0	(43)
Net income applicable to common shareholders	32		(46)	29	(52)
Allstate Life
Segment Reporting Information
Net investment income	123		125	251	252
Realized capital gains (losses), after-tax	16		0	(9)	(4)
Adjusted net income	72		68	152	141
Valuation changes on embedded derivatives not hedged, after-tax	(35)		0	(23)	0
DAC and DSI amortization related to realized capital gains and losses, after-tax	11		(1)	8	(3)
Net income applicable to common shareholders	64		67	128	134
Allstate Benefits
Segment Reporting Information
Net investment income	20		21	40	40
Realized capital gains (losses), after-tax	7		2	(3)	5
Adjusted net income	5		37	29	68
Net income applicable to common shareholders	12		39	26	73
Allstate Annuities
Segment Reporting Information
Net investment income	66		296	113	486
Realized capital gains (losses), after-tax	194		37	(19)	161
Adjusted net income	(111)		52	(250)	27
Valuation changes on embedded derivatives not hedged, after-tax	(6)		(2)	(4)	(5)
Gain on disposition of operations, after-tax	1		1	2	2
Net income applicable to common shareholders	78		88	(271)	185
Corporate and Other
Segment Reporting Information
Realized capital gains (losses), after-tax	23		7	6	8
Adjusted net income	(99)		(98)	(206)	(201)
Pension and other postretirement remeasurement gains (losses), after-tax	(58)		(99)	(309)	(110)
Net income applicable to common shareholders	$ (134)		$ (190)	$ (509)	$ (303)