Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	$ 300,984	$ 317,965
Cash, cash equivalents and short-term investments, Gross unrealized losses	(38)	(120)
Cash, cash equivalents and short-term investments, Fair market value	300,946	317,845
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	143,962	122,420
Cash, cash equivalents and short-term investments, Fair market value	143,962	122,420
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	58,443	69,355
Cash, cash equivalents and short-term investments, Gross unrealized losses	(15)	(51)
Cash, cash equivalents and short-term investments, Fair market value	58,428	69,304
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	79,273	105,340
Cash, cash equivalents and short-term investments, Gross unrealized losses	(19)	(53)
Cash, cash equivalents and short-term investments, Fair market value	79,254	105,287
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	19,306	20,850
Cash, cash equivalents and short-term investments, Gross unrealized losses	(4)	(16)
Cash, cash equivalents and short-term investments, Fair market value	19,302	20,834
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	168,352	122,420
Cash, cash equivalents and short-term investments, Gross unrealized losses	(4)
Cash, cash equivalents and short-term investments, Fair market value	168,348	122,420
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	132,632	195,545
Cash, cash equivalents and short-term investments, Gross unrealized losses	(34)	(120)
Cash, cash equivalents and short-term investments, Fair market value	$ 132,598	$ 195,425