Summary of Company Financial Assets with Maturities of Less Than 90 Days Included in Cash Equivalents (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 419,062	$ 1,147
Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	352,370	$ 1,147
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	16,720
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash equivalents	$ 49,972