Share-Based Compensation - Unrecognized Compensation Expense (Detail) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 96,415
Stock options
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 15,039
Average Expected Recognition Period - in Years	3 years 1 month 6 days
Restricted stock units
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 81,376
Average Expected Recognition Period - in Years	2 years 10 months 28 days