Reinsurance - 2014-1 PCS Excess Agreement (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) contract state
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	$ 500
2014-1 PCS
Ceded Credit Risk [Line Items]
Number of states covered by reinsurance agreement \| state	29
Number of catastrophe reinsurance agreement contracts \| contract	3
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Seventh Layer
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 305
Retention percentage of reinsurance limit	95.00%
Catastrophe reinsurance aggregate annual loss limit	$ 321
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Seventh Layer \| Minimum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	3,070
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Seventh Layer \| Maximum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	3,400
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Eighth Layer
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 115
Retention percentage of reinsurance limit	26.00%
Catastrophe reinsurance aggregate annual loss limit	$ 446
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Eighth Layer \| Minimum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	3,400
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Eighth Layer \| Maximum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	3,840
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Ninth Layer
Ceded Credit Risk [Line Items]
Limit of aggregate losses under aggregate excess agreement	$ 330
Retention percentage of reinsurance limit	57.00%
Catastrophe reinsurance aggregate annual loss limit	$ 578
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Ninth Layer \| Minimum
Ceded Credit Risk [Line Items]
Retention amount for catastrophe reinsurance agreement contracts	3,840
Property Claims Services Catastrophe Reinsurance Agreement Coverage, Ninth Layer \| Maximum
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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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
Balance Type:	debit
Period Type:	duration

- Definition

Represents the retention amount of catastrophe reinsurance agreement contracts.

+ References

No definition available.

+ Details

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- Definition

Number Of Catastrophe Reinsurance Agreement Contracts

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No definition available.

+ Details

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Data Type:	xbrli:integerItemType
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- Definition

Represents the number of states covered under the reinsurance agreement.

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No definition available.

+ Details

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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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- Details

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- Details

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- Details

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- Details

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