Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	$ 617,642,000
Cash and cash equivalents, Gross unrealized gains	1,000
Cash and cash equivalents, Gross unrealized losses	(13,000)
Cash and cash equivalents, Fair market value	617,630,000
Cash, cash equivalents and short-term investments, Amortized cost		$ 317,965,000
Cash, cash equivalents and short-term investments, Gross unrealized losses		(120,000)
Cash, cash equivalents and short-term investments, Fair market value	0	317,845,000
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	420,190,000
Cash and cash equivalents, Fair market value	420,190,000
Cash, cash equivalents and short-term investments, Amortized cost		122,420,000
Cash, cash equivalents and short-term investments, Fair market value		122,420,000
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	83,516,000
Cash and cash equivalents, Gross unrealized losses	(10,000)
Cash and cash equivalents, Fair market value	83,506,000
Cash, cash equivalents and short-term investments, Amortized cost		69,355,000
Cash, cash equivalents and short-term investments, Gross unrealized losses		(51,000)
Cash, cash equivalents and short-term investments, Fair market value		69,304,000
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	113,936,000
Cash and cash equivalents, Gross unrealized gains	1,000
Cash and cash equivalents, Gross unrealized losses	(3,000)
Cash and cash equivalents, Fair market value	113,934,000
Cash, cash equivalents and short-term investments, Amortized cost		105,340,000
Cash, cash equivalents and short-term investments, Gross unrealized losses		(53,000)
Cash, cash equivalents and short-term investments, Fair market value		105,287,000
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents, Amortized cost	617,642,000
Cash and cash equivalents, Gross unrealized gains	1,000
Cash and cash equivalents, Gross unrealized losses	(13,000)
Cash and cash equivalents, Fair market value	$ 617,630,000
Cash, cash equivalents and short-term investments, Amortized cost		122,420,000
Cash, cash equivalents and short-term investments, Fair market value		122,420,000
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		20,850,000
Cash, cash equivalents and short-term investments, Gross unrealized losses		(16,000)
Cash, cash equivalents and short-term investments, Fair market value		20,834,000
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost		195,545,000
Cash, cash equivalents and short-term investments, Gross unrealized losses		(120,000)
Cash, cash equivalents and short-term investments, Fair market value		$ 195,425,000