Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 226,156	$ 419,062
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	211,197	352,370
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 14,959	16,720
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents		$ 49,972