Reportable Segments - Financial performance (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information
Net investment income	$ 824	$ 897	$ 1,610	$ 1,645
Net income applicable to common shareholders	637	550	1,583	1,216
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	419	270	1,381	820
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(3)	(5)	(6)	(7)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	416	265	1,375	813
Net investment income	353	387	690	695
Income tax expense on operations	(157)	(207)	(425)	(475)
Realized capital gains and losses, after-tax	(12)	56	(87)	145
Gain on disposition of operations, after-tax	0	6	0	6
Net income applicable to common shareholders	600	507	1,553	1,184
Service Businesses
Segment Reporting Information
Adjusted net income (loss)	1	(8)	(4)	(18)
Realized capital gains and losses, after-tax	(1)	0	(4)	0
Amortization of purchased intangible assets, after-tax	(16)	(15)	(32)	(30)
Net income applicable to common shareholders	(16)	(23)	(40)	(48)
Allstate Life
Segment Reporting Information
Adjusted net income (loss)	78	63	147	122
Realized capital gains and losses, after-tax	(2)	0	(4)	1
DAC and DSI amortization related to realized capital gains and losses, after-tax	(3)	(3)	(5)	(6)
Net income applicable to common shareholders	73	60	138	117
Allstate Benefits
Segment Reporting Information
Adjusted net income (loss)	34	25	62	47
Realized capital gains and losses, after-tax	0	0	(2)	0
Net income applicable to common shareholders	34	25	60	47
Allstate Annuities
Segment Reporting Information
Adjusted net income (loss)	44	65	79	94
Realized capital gains and losses, after-tax	5	(3)	(18)	(5)
Valuation changes on embedded derivatives not hedged, after-tax	0	(1)	4	(1)
Gain on disposition of operations, after-tax	1	0	2	2
Net income applicable to common shareholders	50	61	67	90
Corporate and Other
Segment Reporting Information
Adjusted net income (loss)	(95)	(80)	(185)	(161)
Realized capital gains and losses, after-tax	(9)	0	(10)	0
Business combination expenses, after-tax	0	0	0	(13)
Net income applicable to common shareholders	$ (104)	$ (80)	$ (195)	$ (174)