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, 2018	Dec. 31, 2017
Financial Assets:
Total financial assets	$ 364,129	$ 515,458
Money market funds
Financial Assets:
Total financial assets	15,062	31,836
Corporate notes and commercial paper
Financial Assets:
Total financial assets	200,431	313,164
U.S. Treasury bills and government agency securities
Financial Assets:
Total financial assets	148,636	170,458
Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	5,186	8,854
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	15,062	31,836
Fair Value, Inputs, Level 1 | Money market funds
Financial Assets:
Total financial assets	15,062	31,836
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	349,067	483,622
Fair Value, Inputs, Level 2 | Corporate notes and commercial paper
Financial Assets:
Total financial assets	200,431	313,164
Fair Value, Inputs, Level 2 | U.S. Treasury bills and government agency securities
Financial Assets:
Total financial assets	148,636	170,458
Fair Value, Inputs, Level 3 | Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	$ 5,186	$ 8,854