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Fair Value of Marketable Securities (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Trading Securities $ 4.8 $ 3.6
Fair Value, Inputs, Level 1    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Trading Securities 4.8 3.6
Fair Value, Inputs, Level 1 | Equity Funds    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Trading Securities   2.1
Fair Value, Inputs, Level 1 | Fixed Income Funds    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Trading Securities   $ 1.5
Fair Value, Inputs, Level 1 | Money Market Funds    
Fair Value Measurement Inputs and Valuation Techniques [Line Items]    
Trading Securities $ 4.8