Fair Value of Financial Assets, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Financial Assets:
Total financial assets	$ 337,383	$ 444,983
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	32,579	22,074
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	304,804	422,909
Money market funds
Financial Assets:
Total financial assets	32,579	22,074
Money market funds | Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	32,579	22,074
Corporate notes and commercial paper
Financial Assets:
Total financial assets	194,158	242,033
Corporate notes and commercial paper | Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	194,158	242,033
U.S. government agency securities
Financial Assets:
Total financial assets	110,646	180,876
U.S. government agency securities | Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	$ 110,646	$ 180,876