Schedule II - Condensed Financial Information of Registrant - Financial Position (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Fixed income securities	$ 66,354	$ 59,044
Short-term investments	7,800	4,256
Cash	377	338
Total assets	125,987	119,950
Liabilities
Long-term debt	7,825	6,631
Total liabilities	95,770	93,952
Shareholders’ equity
Preferred stock and additional capital paid-in, $1 par value, 25 million shares authorized, 81.0 thousand and 92.5 thousand shares issued and outstanding, $2,025 and $2,313 aggregate liquidation preference	1,970	2,248
Common stock, $.01 par value, 2.0 billion shares authorized and 900 million issued, 304 million and 319 million shares outstanding	9	9
Additional capital paid-in	3,498	3,463
Retained income	52,767	48,074
Treasury Stock, Value	(31,331)	(29,746)
Accumulated other comprehensive income:
Unrealized net capital gains and losses	3,180	1,887
Unrealized foreign currency translation adjustments	(7)	(59)
Unamortized pension and other postretirement prior service credit	131	122
Total accumulated other comprehensive income	3,304	1,950
Total shareholders’ equity	30,217	25,998	$ 21,312
Total liabilities and shareholders’ equity	125,987	119,950
Additional balance sheet disclosures:
Other short term Investments, amortized cost	$ 7,800	$ 4,256
Preferred stock, par or stated value per share (in USD per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	81,000	92,500
Preferred stock, shares outstanding (in shares)	81,000.0	92,500
Preferred Stock, liquidation preference	$ 2,025	$ 2,313
Common stock, par or stated value per share (in USD per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	2,000,000,000.0	2,000,000,000.0
Common stock, shares, issued (in shares)	900,000,000	900,000,000
Common stock, shares, outstanding (in shares)	304,000,000	319,000,000
Treasury stock, shares (in shares)	596,000,000	581,000,000
Allstate Corporation
Assets
Investments in subsidiaries	$ 35,603	$ 33,428
Fixed income securities	0	466
Short-term investments	4,479	702
Cash	0	2	$ 0	$ 0
Receivable from subsidiaries	524	448
Deferred income taxes	187	230
Other assets	87	86
Total assets	40,880	35,362
Liabilities
Long-term debt	7,825	6,631
Pension and other postretirement benefit obligations	874	1,081
Deferred compensation	351	327
Payable to subsidiaries	0	14
Notes due to subsidiaries	1,250	1,000
Dividends payable to shareholders	201	199
Other liabilities	162	112
Total liabilities	10,663	9,364
Shareholders’ equity
Preferred stock and additional capital paid-in, $1 par value, 25 million shares authorized, 81.0 thousand and 92.5 thousand shares issued and outstanding, $2,025 and $2,313 aggregate liquidation preference	1,970	2,248
Common stock, $.01 par value, 2.0 billion shares authorized and 900 million issued, 304 million and 319 million shares outstanding	9	9
Additional capital paid-in	3,498	3,463
Retained income	52,767	48,074
Treasury Stock, Value	(31,331)	(29,746)
Accumulated other comprehensive income:
Unrealized net capital gains and losses	3,180	1,887
Unrealized foreign currency translation adjustments	(7)	(59)
Unamortized pension and other postretirement prior service credit	131	122
Total accumulated other comprehensive income	3,304	1,950
Total shareholders’ equity	30,217	25,998
Total liabilities and shareholders’ equity	40,880	35,362
Additional balance sheet disclosures:
AFSs, debt securities, amortized cost	0	458
Other short term Investments, amortized cost	$ 4,479	$ 702
Preferred stock, par or stated value per share (in USD per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	81,000.0	92,500
Preferred stock, shares outstanding (in shares)	81,000.0	92,500
Preferred Stock, liquidation preference	$ 2,025	$ 2,313
Common stock, par or stated value per share (in USD per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	2,000,000,000.0	2,000,000,000.0
Common stock, shares, issued (in shares)	900,000,000	900,000,000
Common stock, shares, outstanding (in shares)	304,000,000	319,000,000
Treasury stock, shares (in shares)	596,000,000	581,000,000