Fair Value of Financial Assets and Liabilities, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Assets:
Total financial assets	$ 562,428	$ 195,289
Money market funds
Financial Assets:
Total financial assets	107,888	6,254
Corporate notes and commercial paper
Financial Assets:
Total financial assets	273,888	133,099
U.S. government agency securities
Financial Assets:
Total financial assets	180,652	55,936
Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	1,512	246
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	107,888	6,254
Fair Value, Inputs, Level 1 | Money market funds
Financial Assets:
Total financial assets	107,888	6,254
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	454,540	189,035
Fair Value, Inputs, Level 2 | Corporate notes and commercial paper
Financial Assets:
Total financial assets	273,888	133,099
Fair Value, Inputs, Level 2 | U.S. government agency securities
Financial Assets:
Total financial assets	180,652	55,936
Fair Value, Inputs, Level 3 | Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	$ 1,512	$ 246