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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - 2024 Convertible Notes [Member] - USD ($)
$ in Millions
Mar. 31, 2019
May 02, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]    
Fair value of liability   $ 136.7
Unamortized debt discount   $ 64.5
Debt instrument interest rate 3.375% 3.375%
Convertible Notes, face value   $ 201.3
Estimated fair value of Convertible Notes $ 174.0