Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information
Net investment income	$ 844	$ 843	$ 2,454	$ 2,488
Tax Legislation expense	31
Net income applicable to common shareholders	833	637	2,416	1,853
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	553	572	1,934	1,392
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(80)	(88)	(86)	(95)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	473	484	1,848	1,297
Net investment income	410	368	1,100	1,063
Income tax expense on operations	(178)	(271)	(603)	(746)
Realized capital gains and losses, after-tax	103	54	16	199
Gain on disposition of operations, after-tax	0	1	0	7
Tax Legislation expense	(3)	0	(3)	0
Net income applicable to common shareholders	805	636	2,358	1,820
Service Businesses
Segment Reporting Information
Adjusted net income (loss)	0	(17)	(4)	(35)
Realized capital gains and losses, after-tax	(1)	0	(5)	0
Amortization of purchased intangible assets, after-tax	(16)	(15)	(48)	(45)
Tax Legislation expense	(4)	0	(4)	0
Net income applicable to common shareholders	(21)	(32)	(61)	(80)
Allstate Life
Segment Reporting Information
Adjusted net income (loss)	74	74	221	196
Realized capital gains and losses, after-tax	(3)	1	(7)	2
DAC and DSI amortization related to realized capital gains and losses, after-tax	(1)	(2)	(6)	(8)
Tax Legislation expense	(16)	0	(16)	0
Net income applicable to common shareholders	54	73	192	190
Allstate Benefits
Segment Reporting Information
Adjusted net income (loss)	32	28	94	75
Realized capital gains and losses, after-tax	2	1	0	1
Net income applicable to common shareholders	34	29	94	76
Allstate Annuities
Segment Reporting Information
Adjusted net income (loss)	20	55	99	149
Realized capital gains and losses, after-tax	40	11	22	6
Valuation changes on embedded derivatives not hedged, after-tax	1	(1)	5	(2)
Gain on disposition of operations, after-tax	1	1	3	3
Tax Legislation expense	69	0	69	0
Net income applicable to common shareholders	131	66	198	156
Corporate and Other
Segment Reporting Information
Adjusted net income (loss)	(155)	(134)	(340)	(295)
Realized capital gains and losses, after-tax	0	0	(10)	0
Tax Legislation expense	(15)	0	(15)	0
Business combination expenses, after-tax	0	(1)	0	(14)
Net income applicable to common shareholders	$ (170)	$ (135)	$ (365)	$ (309)