Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information
Net investment income	$ 832	$ 880	$ 1,662	$ 2,470
Net income applicable to common shareholders	1,126	889	2,863	2,971
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	888	836	3,143	1,909
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(135)	(99)	(141)	(105)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	753	737	3,002	1,804
Net investment income	422	448	802	1,210
Income tax expense on operations	(240)	(236)	(752)	(617)
Realized capital gains (losses), after-tax	230	127	447	724
Net income applicable to common shareholders	1,165	1,076	3,499	3,121
Service Businesses
Segment Reporting Information
Net investment income	12	11	33	30
Realized capital gains (losses), after-tax	11	4	7	17
Adjusted net income	40	8	115	35
Amortization of purchased intangibles, after-tax	(21)	(25)	(63)	(74)
Impairment of purchased intangibles, after-tax	0	0	0	(43)
Net income applicable to common shareholders	30	(13)	59	(65)
Allstate Life
Segment Reporting Information
Net investment income	123	128	374	380
Realized capital gains (losses), after-tax	6	4	(3)	0
Adjusted net income	(14)	44	138	185
Valuation changes on embedded derivatives not hedged, after-tax	(1)	(9)	(24)	(9)
DAC and DSI amortization related to realized capital gains and losses, after-tax	(4)	1	4	(2)
Net income applicable to common shareholders	(13)	40	115	174
Allstate Benefits
Segment Reporting Information
Net investment income	18	21	58	61
Realized capital gains (losses), after-tax	3	2	0	7
Adjusted net income	33	31	62	99
Net income applicable to common shareholders	36	33	62	106
Allstate Annuities
Segment Reporting Information
Net investment income	245	251	358	737
Realized capital gains (losses), after-tax	89	16	70	177
Adjusted net income	37	16	(213)	43
Valuation changes on embedded derivatives not hedged, after-tax	1	(1)	(3)	(6)
Premium deficiency charge for immediate annuities, after-tax	(178)	0	(178)	0
Gain on disposition of operations, after-tax	0	0	2	2
Net income applicable to common shareholders	(51)	31	(322)	216
Corporate and Other
Segment Reporting Information
Realized capital gains (losses), after-tax	7	2	13	10
Adjusted net income	(111)	(101)	(317)	(302)
Pension and other postretirement remeasurement gains (losses), after-tax	56	(179)	(253)	(289)
Curtailment gain, after-tax	7	0	7	0
Net income applicable to common shareholders	(41)	$ (278)	$ (550)	$ (581)
Life premiums | Allstate Annuities
Segment Reporting Information
Premium deficiency charge for immediate annuities, after-tax	$ 225