Fair Value of Financial Assets, and Liabilities, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial Assets:
Total financial assets	$ 195,289	$ 444,983
Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	246
Money market funds
Financial Assets:
Total financial assets	6,254	22,074
Corporate notes and commercial paper
Financial Assets:
Total financial assets	133,099	242,033
U.S. government agency securities
Financial Assets:
Total financial assets	55,936	180,876
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	6,254	22,074
Fair Value, Inputs, Level 1 | Money market funds
Financial Assets:
Total financial assets	6,254	22,074
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	189,035	422,909
Fair Value, Inputs, Level 2 | Corporate notes and commercial paper
Financial Assets:
Total financial assets	133,099	242,033
Fair Value, Inputs, Level 2 | U.S. government agency securities
Financial Assets:
Total financial assets	55,936	$ 180,876
Fair Value, Inputs, Level 3 | Embedded derivative liabilities
Financial Liabilities:
Total financial liabilities	$ 246