Schedule II - Condensed Financial Information of Registrant - Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net income	$ 3,189	$ 1,877	$ 2,171
Adjustments to reconcile net income to net cash provided by operating activities:
Realized capital gains and losses	(445)	90	(30)
Changes in:
Net cash provided by operating activities	4,314	3,993	3,616
Cash flows from investing activities
Change in short-term investments, net	2,610	(2,446)	385
Net cash (used in) provided by investing activities	(1,210)	(2,526)	742
Cash flows from financing activities
Proceeds from issuance of long-term debt	0	1,236	0
Repayments of long-term debt	0	(17)	(20)
Dividends paid on common stock	(525)	(486)	(483)
Dividends paid on preferred stock	(116)	(116)	(116)
Treasury stock purchases	(1,495)	(1,337)	(2,808)
Shares reissued under equity incentive plans, net	135	164	130
Excess tax benefits on share-based payment arrangements	0	32	45
Other	(57)	36	7
Net cash used in financing activities	(2,923)	(1,526)	(4,520)
Net increase (decrease) in cash	181	(59)	(162)
Cash at beginning of year	436	495	657
Cash at end of year	617	436	495
Allstate Corporation
Cash flows from operating activities
Net income	3,189	1,877	2,171
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in net income of subsidiaries	(3,556)	(2,027)	(2,300)
Dividends received from subsidiaries	1,671	1,874	2,300
Realized capital gains and losses	2	(2)	0
Changes in:
Pension and other postretirement benefits	119	10	(15)
Income taxes	35	13	77
Operating assets and liabilities	56	43	26
Net cash provided by operating activities	1,516	1,788	2,259
Cash flows from investing activities
Proceeds from sales of investments	880	389	399
Investment purchases	(748)	(243)	(4)
Investment collections	13	60	0
Return of capital from subsidiaries	42	(1,500)	50
Transfers to subsidiaries through intercompany loan agreement	0	(30)	0
Change in short-term investments, net	48	58	397
Net cash (used in) provided by investing activities	235	(1,266)	842
Cash flows from financing activities
Proceeds from borrowings from subsidiaries	300	0	0
Repayment of notes due to subsidiaries	(50)	0	0
Proceeds from issuance of long-term debt	0	1,236	0
Repayments of long-term debt	0	(17)	(20)
Dividends paid on common stock	(525)	(486)	(483)
Dividends paid on preferred stock	(116)	(116)	(116)
Treasury stock purchases	(1,495)	(1,337)	(2,808)
Shares reissued under equity incentive plans, net	135	164	130
Excess tax benefits on share-based payment arrangements	0	32	45
Other	(2)	0	0
Net cash used in financing activities	(1,753)	(524)	(3,252)
Net increase (decrease) in cash	(2)	(2)	(151)
Cash at beginning of year	2	4	155
Cash at end of year	$ 0	$ 2	$ 4