Schedule II - Condensed Financial Information of Registrant - Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
	Oct. 15, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income		$ 4,847	$ 2,160	$ 3,554
Adjustments to reconcile net income to net cash provided by operating activities:
Realized capital gains and losses		(1,885)	877	(445)
Changes in:
Net cash provided by operating activities		5,129	5,175	4,314
Cash flows from investing activities
Change in short-term investments, net		(767)	(505)	2,610
Net cash used in investing activities		(2,807)	(1,719)	(1,210)
Cash flows from financing activities
Proceeds from issuance of long-term debt		491	498	0
Redemption of preferred stock	$ (1,130)	(1,132)	(385)	0
Redemption and repayment of long-term debt		(317)	(400)	0
Proceeds from issuance of preferred stock		1,414	557	0
Dividends paid on common stock		(653)	(614)	(525)
Dividends paid on preferred stock		(134)	(134)	(116)
Treasury stock purchases		(1,735)	(2,303)	(1,495)
Shares reissued under equity incentive plans, net		120	73	135
Other		129	91	(57)
Net cash used in financing activities		(2,483)	(3,574)	(2,923)
Cash at beginning of year		499
Cash at end of year		338	499
Allstate Corporation
Cash flows from operating activities
Net income		4,847	2,160	3,554
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries		(5,072)	(2,731)	(3,556)
Dividends received from subsidiaries		2,434	2,059	1,671
Realized capital gains and losses		(9)	10	2
Pension and other postretirement remeasurement gains and losses		103	454	(219)
Changes in:
Pension and other postretirement benefits		(122)	(116)	(224)
Income taxes		13	(28)	232
Operating assets and liabilities		111	160	56
Net cash provided by operating activities		2,305	1,968	1,516
Cash flows from investing activities
Proceeds from sales of investments		1,094	1,370	880
Proceeds from sales of investments to subsidiaries		0	390	0
Investment purchases		(892)	(1,037)	(748)
Investment collections		65	108	13
Capital contribution or return of capital from subsidiaries		43	(975)	42
Change in short-term investments, net		(417)	(115)	48
Net cash used in investing activities		(107)	(259)	235
Cash flows from financing activities
Proceeds from borrowings from subsidiaries		1,000	1,250	300
Repayment of notes due to subsidiaries		(1,250)	(250)	(50)
Proceeds from issuance of long-term debt		491	498	0
Redemption of preferred stock		(1,132)	(385)	0
Redemption and repayment of long-term debt		(317)	(400)	0
Proceeds from issuance of preferred stock		1,414	557	0
Dividends paid on common stock		(653)	(614)	(525)
Dividends paid on preferred stock		(134)	(134)	(116)
Treasury stock purchases		(1,735)	(2,303)	(1,495)
Shares reissued under equity incentive plans, net		120	73	135
Other		0	(1)	(2)
Net cash used in financing activities		(2,196)	(1,709)	(1,753)
Net increase (decrease) in cash		2	0	(2)
Cash at beginning of year		0	0	2
Cash at end of year		$ 2	$ 0	$ 0