Reinsurance - 2017-1 Excess Catastrophe Reinsurance Contract (Details) $ in Millions	9 Months Ended	12 Months Ended
Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) state
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts		$ 500
2017-1 Excess Catastrophe Reinsurance Contract
Ceded Credit Risk [Line Items]
Number of states covered by reinsurance agreement \| state		48
Limit of aggregate losses under aggregate excess agreement	$ 375
Retention percentage of reinsurance limit	37.00%
Catastrophe reinsurance aggregate annual loss limit	$ 1,020
Minimum \| 2017-1 Excess Catastrophe Reinsurance Contract
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	3,400
Maximum \| 2017-1 Excess Catastrophe Reinsurance Contract
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	$ 4,420

- Definition

Represents the retention percentage of annual coverage limit provided by the insurance arrangement for catastrophe claims.

+ References

No definition available.

+ Details

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- Definition

Catastrophe Reinsurance Aggregate Annual Loss Limit

+ References

No definition available.

+ Details

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Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

The limit of aggregated losses per contract year beyond which coverage provided by an aggregate excess agreement applies.

+ References

No definition available.

+ Details

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Namespace Prefix:	all_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

- Definition

Represents the retention amount of catastrophe reinsurance agreement contracts.

+ References

No definition available.

+ Details

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- Definition

Represents the number of states covered under the reinsurance agreement.

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No definition available.

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- 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

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- Details

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- Details

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