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Employee Benefit Plans Assets Held in Deferral Plan Irrevocable Grantor Trust Account (Detail) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 295.2	$ 259.1
Progressive common shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance	4.4	4.7
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 128.2	$ 122.2
Other Investment Funds
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 167.0	$ 136.9

X

- Definition

The maximum number of shares authorized for issuance under the deferred compensation arrangement as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

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X

- Definition

Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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X

- 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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