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Financial Instruments - Valuation of Financial Instruments Recorded on Fair Value Basis (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Noncurrent marketable securities $ 4.0 $ 10.7
Fair Value Measurements Recurring    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Noncurrent marketable securities 4.0 10.7
Fair Value Measurements Recurring | Level 1    
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Noncurrent marketable securities $ 4.0 $ 10.7