Fair Value Measurement - Summary of Fair Value Measurements of Cash Equivalents, Available-for-Sale Securities and Contingent Consideration Liability (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents and marketable securities	$ 223,379	$ 139,466
Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	24,290	18,992
Commercial paper securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	4,595
Fair value on recurring basis [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	28,885	18,992
Total marketable securities	194,494	120,474
Total cash equivalents and marketable securities	223,379	139,466
Fair value on recurring basis [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	24,290	18,992
Fair value on recurring basis [Member] | Commercial paper securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	4,595
Total marketable securities	110,247	23,185
Fair value on recurring basis [Member] | Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	24,290	18,992
Total cash equivalents and marketable securities	24,290	18,992
Fair value on recurring basis [Member] | Level 1 [Member] | Money market funds [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	24,290	18,992
Fair value on recurring basis [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	4,595
Total marketable securities	194,494	120,474
Total cash equivalents and marketable securities	199,089	120,474
Fair value on recurring basis [Member] | Level 2 [Member] | Commercial paper securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total cash equivalents	4,595
Total marketable securities	110,247	23,185
Fair value on recurring basis [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total marketable securities	75,755	10,004
Fair value on recurring basis [Member] | Corporate debt securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total marketable securities	75,755	10,004
Fair value on recurring basis [Member] | U.S. government sponsored entity debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total marketable securities	8,492	87,285
Fair value on recurring basis [Member] | U.S. government sponsored entity debt securities [Member] | Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total marketable securities	$ 8,492	$ 87,285