Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information
Net investment income	$ 942	$ 824	$ 1,590	$ 1,610
Net income applicable to common shareholders	821	678	2,082	1,655
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	370	458	1,073	1,466
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(3)	(3)	(6)	(6)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	367	455	1,067	1,460
Net investment income	471	353	762	690
Income tax expense on operations	(179)	(166)	(381)	(443)
Realized capital gains and losses, after-tax	204	(12)	597	(87)
Net income applicable to common shareholders	863	630	2,045	1,620
Service Businesses
Segment Reporting Information
Adjusted net income (loss)	16	2	27	(1)
Realized capital gains and losses, after-tax	6	(1)	13	(4)
Amortization of purchased intangibles, after-tax	(25)	(16)	(49)	(32)
Impairment of purchased intangibles, after-tax	(43)	0	(43)	0
Net income applicable to common shareholders	(46)	(15)	(52)	(37)
Allstate Life
Segment Reporting Information
Adjusted net income (loss)	68	80	141	151
Realized capital gains and losses, after-tax	0	(2)	(4)	(4)
DAC and DSI amortization related to realized capital gains and losses, after-tax	(1)	(3)	(3)	(5)
Net income applicable to common shareholders	67	75	134	142
Allstate Benefits
Segment Reporting Information
Adjusted net income (loss)	37	36	68	65
Realized capital gains and losses, after-tax	2	0	5	(2)
Net income applicable to common shareholders	39	36	73	63
Allstate Annuities
Segment Reporting Information
Adjusted net income (loss)	52	44	27	79
Realized capital gains and losses, after-tax	37	5	161	(18)
Valuation changes on embedded derivatives not hedged, after-tax	(2)	0	(5)	4
Gain on disposition of operations, after-tax	1	1	2	2
Net income applicable to common shareholders	88	50	185	67
Corporate and Other
Segment Reporting Information
Adjusted net income (loss)	(98)	(95)	(201)	(185)
Realized capital gains and losses, after-tax	7	(9)	8	(10)
Pension and other postretirement remeasurement gains and losses, after-tax	(99)	6	(110)	(5)
Net income applicable to common shareholders	$ (190)	$ (98)	$ (303)	$ (200)