Equity Incentive Plans - Narrative (Details) - shares shares in Millions		12 Months Ended
Feb. 17, 2014	Dec. 31, 2020
Equity Incentive Plans
Common stock authorized for stock-based awards (in shares)		110.8
Common stock reserved for further issuance (in shares)		20.9
Stock options
Equity Incentive Plans
Vesting period of options granted		3 years
Portion of awards vesting and unrestricting on second anniversary of grant date (as a percent)	50.00%
Portion of awards vesting and unrestricting on third and fourth anniversaries of grant date (as a percent)	25.00%
Expiration term for options		10 years

Represents the number of years after which granted options expire.

No definition available.

Name:	all_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardExpirationTerm
Namespace Prefix:	all_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of share-based awards that vest on the second anniversary of the grant date of the awards.

+ References

No definition available.

+ Details

Name:	all_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPercentSecondAnniversary
Namespace Prefix:	all_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of share-based awards that vest on the third and fourth anniversaries of the grant date of the awards.

+ References

No definition available.

+ Details

Name:	all_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPercentThirdFourthAnniversary
Namespace Prefix:	all_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The period of time over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, for awards granted prior to 2010.

+ References

No definition available.

+ Details

Name:	all_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriodBefore2010
Namespace Prefix:	all_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: