Share-Based Compensation Unrecognized Share-Based Compensation Expense (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 60,160
Stock option
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 9,630
Average Expected Recognition Period - in Years	2 years 7 months 3 days
Restricted stock units
Unrecognized Share Based Compensation Expense [Line Items]
Unrecognized Expense	$ 50,530
Average Expected Recognition Period - in Years	2 years 7 months 3 days