Schedule II - Condensed Financial Information of Registrant - Financial Position (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Fixed income securities, at fair value (amortized cost $458 and $355)	$ 59,044	$ 57,170
Short-term investments	4,256	3,027
Cash	338	499
Total assets	119,950	112,249
Liabilities
Long-term debt	6,631	6,451
Total liabilities	93,952	90,937
Shareholders’ equity
Preferred stock and additional capital paid-in, $1 par value, 25 million shares authorized, 92.5 thousand and 79.8 thousand shares issued and outstanding, $2,313 and $1,995 aggregate liquidation preference	2,248	1,930
Common stock, $.01 par value, 2.0 billion shares authorized and 900 million issued, 319 million and 332 million shares outstanding	9	9
Additional capital paid-in	3,463	3,310
Retained income	48,074	44,033
Deferred Employee Stock Ownership Plan (“ESOP”) expense	0	(3)
Treasury stock, at cost (581 million and 568 million shares)	(29,746)	(28,085)
Accumulated other comprehensive income:
Unrealized net capital gains and losses	1,887	(2)
Unrealized foreign currency translation adjustments	(59)	(49)
Unamortized pension and other postretirement prior service credit	122	169
Total accumulated other comprehensive income	1,950	118
Total shareholders’ equity	25,998	21,312	$ 22,551
Total liabilities and shareholders’ equity	119,950	112,249
Additional balance sheet disclosures:
Short-term, at fair value, amortized cost (in dollars)	$ 4,256	$ 3,027
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	92,500	79,800
Preferred stock, shares outstanding (in shares)	92,500	79,800
Preferred stock, shares aggregate liquidation preference (in dollars)	$ 2,313	$ 1,995
Common stock, par value (in dollars 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)	319,000,000	332,000,000
Treasury stock, shares (in shares)	581,000,000	568,000,000
Allstate Corporation
Assets
Investments in subsidiaries	$ 33,428	$ 29,301
Fixed income securities, at fair value (amortized cost $458 and $355)	466	356
Short-term investments	702	285
Cash	2	0	$ 0	$ 2
Receivable from subsidiaries	448	426
Deferred income taxes	230	225
Other assets	86	92
Total assets	35,362	30,685
Liabilities
Long-term debt	6,631	6,451
Pension and other postretirement benefit obligations	1,081	1,050
Deferred compensation	327	281
Payable to subsidiaries	14	3
Notes due to subsidiaries	1,000	1,250
Dividends payable to shareholders	199	198
Other liabilities	112	140
Total liabilities	9,364	9,373
Shareholders’ equity
Preferred stock and additional capital paid-in, $1 par value, 25 million shares authorized, 92.5 thousand and 79.8 thousand shares issued and outstanding, $2,313 and $1,995 aggregate liquidation preference	2,248	1,930
Common stock, $.01 par value, 2.0 billion shares authorized and 900 million issued, 319 million and 332 million shares outstanding	9	9
Additional capital paid-in	3,463	3,310
Retained income	48,074	44,033
Deferred Employee Stock Ownership Plan (“ESOP”) expense	0	(3)
Treasury stock, at cost (581 million and 568 million shares)	(29,746)	(28,085)
Accumulated other comprehensive income:
Unrealized net capital gains and losses	1,887	(2)
Unrealized foreign currency translation adjustments	(59)	(49)
Unamortized pension and other postretirement prior service credit	122	169
Total accumulated other comprehensive income	1,950	118
Total shareholders’ equity	25,998	21,312
Total liabilities and shareholders’ equity	35,362	30,685
Additional balance sheet disclosures:
Fixed income securities, at fair value, amortized cost (in dollars)	458	355
Short-term, at fair value, amortized cost (in dollars)	$ 702	$ 285
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Preferred stock, shares authorized (in shares)	25,000,000	25,000,000
Preferred stock, shares issued (in shares)	92,500	79,800
Preferred stock, shares outstanding (in shares)	92,500	79,800
Preferred stock, shares aggregate liquidation preference (in dollars)	$ 2,313	$ 1,995
Common stock, par value (in dollars 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)	319,000,000	332,000,000
Treasury stock, shares (in shares)	581,000,000	568,000,000