Net Loss Per Common Share - Additional Information (Detail)	Jun. 30, 2018 $ / shares
Convertible Notes due 2018
Earnings Per Share [Line Items]
Debt instrument, interest rate, stated percentage, per annum	0.75%
Percentage of capped call transactions to principal amount	50.00%
Convertible Notes due 2020
Earnings Per Share [Line Items]
Debt instrument, interest rate, stated percentage, per annum	1.50%
Percentage of capped call transactions to principal amount	50.00%
Convertible Notes due 2018 and due 2020
Earnings Per Share [Line Items]
Debt instrument, convertible, conversion price, per share	$ 94.15