Fair Value of Financial Assets and Liabilities, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Assets:
Total financial assets	$ 434,869	$ 515,458
Money market funds
Financial Assets:
Total financial assets	19,507	31,836
Corporate notes and commercial paper
Financial Assets:
Total financial assets	260,950	313,164
U.S. Treasury bills and government agency securities
Financial Assets:
Total financial assets	154,412	170,458
Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	7,220	8,854
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	19,507	31,836
Fair Value, Inputs, Level 1 | Money market funds
Financial Assets:
Total financial assets	19,507	31,836
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	415,362	483,622
Fair Value, Inputs, Level 2 | Corporate notes and commercial paper
Financial Assets:
Total financial assets	260,950	313,164
Fair Value, Inputs, Level 2 | U.S. Treasury bills and government agency securities
Financial Assets:
Total financial assets	154,412	170,458
Fair Value, Inputs, Level 3 | Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	$ 7,220	$ 8,854