Fair Value of Financial Assets, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financial Assets:
Total financial assets	$ 243,058	$ 444,983
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	3,003	22,074
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	240,055	422,909
Money market funds
Financial Assets:
Total financial assets	3,003	22,074
Money market funds | Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	3,003	22,074
Corporate notes and commercial paper
Financial Assets:
Total financial assets	162,819	242,033
Corporate notes and commercial paper | Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	162,819	242,033
U.S. government agency securities
Financial Assets:
Total financial assets	77,236	180,876
U.S. government agency securities | Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	$ 77,236	$ 180,876