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Fair Value of Financial Assets and Liabilities - Additional Information (Detail) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
May 02, 2017
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Transfers between Level 1, Level 2 and Level 3 $ 0 $ 0  
2024 Convertible Notes [Member]      
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]      
Fair value of liability     $ 136,700,000
Unamortized debt discount     $ 64,500,000
Debt instrument interest rate 3.375%   3.375%
Convertible Notes, face value     $ 201,300,000
Estimated fair value of Convertible Notes $ 245,800,000