Fair Value Measurements - Schedule of Fair Value of Financial Instruments (Detail) - Recurring - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	$ 152,597	$ 182,665
Total liabilities measured at fair value	2,233	2,022
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	55,480	60,639
U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	81,175	81,079
U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	15,942	40,947
Derivative liability associated with Medicis settlement
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	2,233	2,022
Level 1
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	136,655	141,718
Total liabilities measured at fair value	0	0
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	55,480	60,639
Level 1 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	81,175	81,079
Level 1 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 1 | Derivative liability associated with Medicis settlement
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	15,942	40,947
Total liabilities measured at fair value	0	0
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 2 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 2 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	15,942	40,947
Level 2 | Derivative liability associated with Medicis settlement
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Total liabilities measured at fair value	2,233	2,022
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | U.S. treasury securities
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | U.S. government agency obligations
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total assets measured at fair value	0	0
Level 3 | Derivative liability associated with Medicis settlement
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total liabilities measured at fair value	$ 2,233	$ 2,022