Reporting Segments - Reportable segments financial performance (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information
Net investment income										$ 3,159	$ 3,240	$ 3,401
Goodwill impairment									$ (125)	(106)	0	(125)
Net income applicable to common shareholders	$ 1,707	$ 889	$ 821	$ 1,261	$ (585)	$ 942	$ 678	$ 977		4,678	2,012	3,438
Allstate Protection
Segment Reporting Information
Underwriting income										2,912	2,343	2,304
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income										(108)	(90)	(99)
Property-Liability
Segment Reporting Information
Underwriting income										2,804	2,253	2,205
Net investment income										1,533	1,464	1,478
Income tax expense on operations										(887)	(747)	(1,187)
Realized capital gains and losses, after-tax										1,161	(500)	272
Gain (loss) on disposition of operations, after-tax										0	0	9
Tax Legislation (expense) benefit										0	(5)	36
Net income applicable to common shareholders										4,611	2,465	2,813
Service Businesses
Segment Reporting Information
Net investment income										42	27	16
Realized capital gains and losses, after-tax										25	(9)	0
Tax Legislation (expense) benefit										0	(4)	137
Net income applicable to common shareholders										(77)	(79)	23
Adjusted net income (loss)										38	8	(54)
Amortization of purchased intangibles, after-tax										(97)	(74)	(60)
Impairment of Intangible Assets (Excluding Goodwill), Net of Tax										(43)	0	0
Allstate Life
Segment Reporting Information
Net investment income										514	505	489
Realized capital gains and losses, after-tax										0	(11)	2
Tax Legislation (expense) benefit										0	(16)	338
Net income applicable to common shareholders										247	260	589
Adjusted net income (loss)										261	295	259
Valuation changes on embedded derivatives that are not hedged, after-tax										(9)	0	0
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax										(5)	(8)	(10)
Allstate Benefits
Segment Reporting Information
Net investment income										83	77	72
Realized capital gains and losses, after-tax										9	(7)	0
Tax Legislation (expense) benefit										0	0	54
Net income applicable to common shareholders										124	118	154
Adjusted net income (loss)										115	124	100
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax										0	1	0
Allstate Annuities
Segment Reporting Information
Net investment income										917	1,096	1,305
Realized capital gains and losses, after-tax										274	(131)	28
Gain (loss) on disposition of operations, after-tax										4	4	4
Tax Legislation (expense) benefit										0	69	182
Net income applicable to common shareholders										282	76	419
Adjusted net income (loss)										10	131	205
Valuation changes on embedded derivatives that are not hedged, after-tax										(6)	3	0
Corporate and Other
Segment Reporting Information
Net investment income										70	71	41
Realized capital gains and losses, after-tax										19	(30)	(4)
Pension and other postretirement remeasurement gains and losses, after-tax										(90)	(370)	141
Tax Legislation (expense) benefit										0	(15)	(238)
Goodwill impairment										0	0	(125)
Business combination expenses, after-tax										0	(7)	(14)
Net income applicable to common shareholders										(509)	(828)	(560)
Adjusted net income (loss)										$ (438)	$ (406)	$ (320)