Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 419,484	$ 419,062
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	370,052	352,370
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	24,451	16,720
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 24,981	$ 49,972