Fair Value Measurements (Detail) - Fair value, measurements, recurring - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	$ 333,662	$ 75,650
Fair value assets and liabilities measured on recurring basis, total	333,662	75,650
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	333,662	75,650
Fair value assets and liabilities measured on recurring basis, total	333,662	75,650
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	0	0
Fair value assets and liabilities measured on recurring basis, total	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents (money market accounts)	0	0
Fair value assets and liabilities measured on recurring basis, total	$ 0	$ 0