Employee Benefit Plans Assets Held in Deferral Plan Irrevocable Grantor Trust Account (Detail) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 272.8	$ 238.7
Other Investment Funds
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	151.2	124.0
Common Stock [Member] \| Progressive common shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 121.6	$ 114.7
Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance	3.2	3.6

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

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=116856206&loc=d3e5070-113901

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

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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.

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