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Fair Value of Marketable Securities (Detail) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020
--- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | $ 5.6 | $ 5.7 | $ 5.7
Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 0.5 | 0.5 | 0.5
Fair Value, Inputs, Level 1 |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 5.6 | 5.7 | 5.7
Fair Value, Inputs, Level 1 | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 0.5 | 0.5 | 0.5
Fair Value, Inputs, Level 1 | Equity Funds | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 0.1 | 0.2 | 0.2
Fair Value, Inputs, Level 1 | Money Market Funds |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 5.6 | 5.7 | 5.7
Fair Value, Inputs, Level 1 | Money Market Funds | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | $ 0.4 | $ 0.3 | $ 0.3

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 + Details
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