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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

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- 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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- 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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