Share-Based Compensation - Unrecognized Compensation Expense (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 54,326
Stock options
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 8,290
Average Expected Recognition Period - in Years	2 years 5 months 23 days
Restricted stock units
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 46,036
Average Expected Recognition Period - in Years	2 years 5 months 23 days