Share-Based Compensation - Unrecognized Compensation Expense (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 86,562
Stock options
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 13,525
Average Expected Recognition Period - in Years	2 years 10 months 28 days
Restricted stock units
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 73,037
Average Expected Recognition Period - in Years	2 years 8 months 27 days