Schedule II - Condensed Financial Information of Registrant - Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
	Jan. 15, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income			$ 5,576	$ 4,847	$ 2,160
Adjustments to reconcile net income to net cash provided by operating activities:
Realized capital gains (losses)		$ (4)	(1,356)	(1,885)	877
Pension and other postretirement remeasurement (gains) losses			51	(114)	(468)
Changes in:
Net cash provided by operating activities			5,491	5,129	5,175
Cash flows from investing activities
Change in short-term and other investments, net			(3,871)	(725)	(603)
Net cash used in investing activities			(3,441)	(2,807)	(1,719)
Cash flows from financing activities
Proceeds from issuance of long-term debt			1,189	491	498
Redemption of preferred stock	$ (288)
Redemption and repayment of long-term debt			0	(317)	(400)
Proceeds from issuance of preferred stock			0	1,414	557
Dividends paid on common stock			(668)	(653)	(614)
Dividends paid on preferred stock			(108)	(134)	(134)
Treasury stock purchases			(1,737)	(1,735)	(2,303)
Shares reissued under equity incentive plans, net			63	120	73
Other			41	129	91
Net cash used in financing activities			(2,011)	(2,483)	(3,574)
Cash at beginning of year			338
Cash at end of year		377	377	338
Allstate Corporation
Cash flows from operating activities
Net income			5,576	4,847	2,160
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries			(5,665)	(5,072)	(2,731)
Dividends received from subsidiaries			4,157	2,434	2,059
Realized capital gains (losses)			(33)	(9)	10
Pension and other postretirement remeasurement (gains) losses			(73)	103	454
Changes in:
Pension and other postretirement benefits			(168)	(122)	(116)
Income taxes			54	13	(28)
Operating assets and liabilities			110	111	160
Net cash provided by operating activities			3,958	2,305	1,968
Cash flows from investing activities
Proceeds from sales of investments			1,251	1,094	1,370
Proceeds from sales of investments to subsidiaries			0	0	390
Investment purchases			(402)	(892)	(1,037)
Investment collections			16	65	108
Capital contribution or return of capital from subsidiaries			251	43	(975)
Change in short-term and other investments, net			(3,777)	(417)	(115)
Net cash used in investing activities			(2,661)	(107)	(259)
Cash flows from financing activities
Proceeds from borrowings from subsidiaries			1,250	1,000	1,250
Repayment of notes due to subsidiaries			(1,000)	(1,250)	(250)
Proceeds from issuance of long-term debt			1,189	491	498
Redemption of preferred stock			(288)	(1,132)	(385)
Redemption and repayment of long-term debt			0	(317)	(400)
Proceeds from issuance of preferred stock			0	1,414	557
Dividends paid on common stock			(668)	(653)	(614)
Dividends paid on preferred stock			(108)	(134)	(134)
Treasury stock purchases			(1,737)	(1,735)	(2,303)
Shares reissued under equity incentive plans, net			63	120	73
Other			0	0	(1)
Net cash used in financing activities			(1,299)	(2,196)	(1,709)
Net (decrease) increase in cash			(2)	2	0
Cash at beginning of year			2	0	0
Cash at end of year		$ 0	$ 0	$ 2	$ 0