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Fair Value Measurements - Additional Information (Detail) - 1.75% Convertible Senior Notes due 2021 [Member] - USD ($)
$ in Millions
1 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2016
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Debt instrument interest rate 1.75% 1.75% 1.75%
Debt instrument due year 2021 2021 2021
Fair value of notes   $ 187.9 $ 180.3