Summary of Company Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	$ 99,910	$ 210,439
Cash, cash equivalents and short-term investments, Gross unrealized gains	1
Cash, cash equivalents and short-term investments, Gross unrealized losses	(2)	(95)
Cash, cash equivalents and short-term investments, Fair market value	99,909	210,344
Cash and Cash Equivalents [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	55,819	73,551
Cash, cash equivalents and short-term investments, Fair market value	55,819	73,551
Short-term Investments [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	44,091	136,888
Cash, cash equivalents and short-term investments, Gross unrealized gains	1
Cash, cash equivalents and short-term investments, Gross unrealized losses	(2)	(95)
Cash, cash equivalents and short-term investments, Fair market value	44,090	136,793
Cash and Money Market Funds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	44,720	73,551
Cash, cash equivalents and short-term investments, Fair market value	44,720	73,551
Commercial Paper [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	11,099	2,997
Cash, cash equivalents and short-term investments, Fair market value	11,099	2,997
Government and Government Agency Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	28,248	75,289
Cash, cash equivalents and short-term investments, Gross unrealized gains	1
Cash, cash equivalents and short-term investments, Gross unrealized losses		(39)
Cash, cash equivalents and short-term investments, Fair market value	28,249	75,250
Corporate Bonds [Member]
Cash and Cash Equivalents [Line Items]
Cash, cash equivalents and short-term investments, Amortized cost	15,843	58,602
Cash, cash equivalents and short-term investments, Gross unrealized losses	(2)	(56)
Cash, cash equivalents and short-term investments, Fair market value	$ 15,841	$ 58,546