Reportable Segments (Details 2) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Property-Liability
Net investment income	$ 843	$ 748	$ 2,488	$ 2,241
Restructuring and related charges	(14)	(5)	(77)	(21)
Allstate Financial
Life and annuity premiums and contract charges	593	571	1,777	1,701
Corporate and Other
Net income applicable to common shareholders	637	491	1,853	950
Allstate Protection
Property-Liability
Underwriting income (loss)	517	455	1,258	518
Discontinued Lines and Coverages
Property-Liability
Underwriting income (loss)	(88)	(100)	(95)	(104)
Property-Liability
Property-Liability
Underwriting income (loss)	429	355	1,163	414
Net investment income	372	310	1,074	928
Income tax (expense) benefit on operations	(252)	(218)	(703)	(429)
Realized capital gains and losses, after-tax	54	36	199	(10)
Allstate Financial
Realized capital gains and losses, after-tax	54	36	199	(10)
Gain on disposition of operations, after-tax	1	0	7	0
Corporate and Other
Realized capital gains and losses, after-tax	54	36	199	(10)
Net income applicable to common shareholders	604	483	1,740	903
Allstate Financial
Property-Liability
Net investment income	461	427	1,383	1,281
Restructuring and related charges	(1)	0	(2)	(1)
Income tax (expense) benefit on operations	(79)	(43)	(206)	(147)
Realized capital gains and losses, after-tax	13	(14)	9	(46)
Allstate Financial
Life and annuity premiums and contract charges	593	571	1,777	1,701
Contract benefits and interest credited to contractholder funds	(629)	(667)	(1,935)	(1,939)
Operating costs and expenses and amortization of deferred policy acquisition costs	(188)	(194)	(597)	(577)
Operating income (loss)	157	94	420	318
Realized capital gains and losses, after-tax	13	(14)	9	(46)
Valuation changes on embedded derivatives that are not hedged, after-tax	(1)	0	(2)	(8)
DAC and DSI amortization related to realized capital gains and losses and valuation changes on embedded derivatives that are not hedged, after-tax	(2)	(1)	(8)	(3)
Gain on disposition of operations, after-tax	1	1	3	3
Corporate and Other
Operating income (loss)	157	94	420	318
Realized capital gains and losses, after-tax	13	(14)	9	(46)
Net income applicable to common shareholders	168	80	422	264
Corporate and Other
Property-Liability
Net investment income	10	11	31	32
Income tax (expense) benefit on operations	60	26	121	77
Realized capital gains and losses, after-tax	0	0	0	(1)
Allstate Financial
Operating income (loss)	(134)	(72)	(295)	(216)
Realized capital gains and losses, after-tax	0	0	0	(1)
Corporate and Other
Operating costs and expenses	(175)	(80)	(360)	(238)
Preferred stock dividends	(29)	(29)	(87)	(87)
Operating income (loss)	(134)	(72)	(295)	(216)
Realized capital gains and losses, after-tax	0	0	0	(1)
Business combination expenses, after-tax	(1)	0	(14)	0
Net income applicable to common shareholders	$ (135)	$ (72)	$ (309)	$ (217)