Reportable Segments - Financial performance (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information
Net investment income	$ 880,000,000	$ 844,000,000	$ 2,470,000,000	$ 2,454,000,000
Net income applicable to common shareholders	889,000,000	942,000,000	2,971,000,000	2,597,000,000
Allstate Protection
Segment Reporting Information
Underwriting income (loss)	836,000,000	587,000,000	1,909,000,000	2,053,000,000
Discontinued Lines and Coverages
Segment Reporting Information
Underwriting income (loss)	(99,000,000)	(80,000,000)	(105,000,000)	(86,000,000)
Property-Liability
Segment Reporting Information
Underwriting income (loss)	737,000,000	507,000,000	1,804,000,000	1,967,000,000
Net investment income	448,000,000	410,000,000	1,210,000,000	1,100,000,000
Income tax expense on operations	(236,000,000)	(185,000,000)	(617,000,000)	(628,000,000)
Realized capital gains and losses, after-tax	127,000,000	103,000,000	724,000,000	16,000,000
Tax Legislation expense (benefit)	0	(3,000,000)	0	(3,000,000)
Net income applicable to common shareholders	1,076,000,000	832,000,000	3,121,000,000	2,452,000,000
Service Businesses
Segment Reporting Information
Adjusted net income	8,000,000	1,000,000	35,000,000	0
Realized capital gains and losses, after-tax	4,000,000	(1,000,000)	17,000,000	(5,000,000)
Amortization of purchased intangibles, after-tax	(25,000,000)	(16,000,000)	(74,000,000)	(48,000,000)
Impairment of purchased intangibles, after-tax	0		(43,000,000)	0
Tax Legislation expense (benefit)	0	(4,000,000)	0	(4,000,000)
Net income applicable to common shareholders	(13,000,000)	(20,000,000)	(65,000,000)	(57,000,000)
Allstate Life
Segment Reporting Information
Adjusted net income	44,000,000	75,000,000	185,000,000	226,000,000
Realized capital gains and losses, after-tax	4,000,000	(3,000,000)	0	(7,000,000)
Valuation changes on embedded derivatives not hedged, after-tax	(9,000,000)	0	(9,000,000)
DAC and DSI amortization related to realized capital gains and losses, after-tax	1,000,000	(1,000,000)	(2,000,000)	(6,000,000)
Tax Legislation expense (benefit)	0	(16,000,000)	0	(16,000,000)
Net income applicable to common shareholders	40,000,000	55,000,000	174,000,000	197,000,000
Allstate Benefits
Segment Reporting Information
Adjusted net income	31,000,000	33,000,000	99,000,000	98,000,000
Realized capital gains and losses, after-tax	2,000,000	2,000,000	7,000,000	0
Net income applicable to common shareholders	33,000,000	35,000,000	106,000,000	98,000,000
Allstate Annuities
Segment Reporting Information
Adjusted net income	16,000,000	20,000,000	43,000,000	99,000,000
Realized capital gains and losses, after-tax	16,000,000	40,000,000	177,000,000	22,000,000
Valuation changes on embedded derivatives not hedged, after-tax	(1,000,000)	1,000,000	(6,000,000)	5,000,000
Gain on disposition of operations, after-tax	0	1,000,000	2,000,000	3,000,000
Tax Legislation expense (benefit)	0	69,000,000	0	69,000,000
Net income applicable to common shareholders	31,000,000	131,000,000	216,000,000	198,000,000
Corporate and Other
Segment Reporting Information
Adjusted net income	(101,000,000)	(106,000,000)	(302,000,000)	(291,000,000)
Realized capital gains and losses, after-tax	2,000,000	0	10,000,000	(10,000,000)
Pension and other postretirement remeasurement gains and losses, after-tax	(179,000,000)	30,000,000	(289,000,000)	25,000,000
Tax Legislation expense (benefit)	0	(15,000,000)	0	(15,000,000)
Net income applicable to common shareholders	$ (278,000,000)	$ (91,000,000)	$ (581,000,000)	$ (291,000,000)