Fair Value of Financial Assets, and Liabilities, Allocated into Level 1, Level 2, and Level 3 Measured on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financial Assets:
Total financial assets	$ 442,509	$ 371,131
Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	55,862	24,915
Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	386,647	346,216
Money market funds
Financial Assets:
Total financial assets	55,862	24,915
Money market funds | Fair Value, Inputs, Level 1
Financial Assets:
Total financial assets	55,862	24,915
Corporate notes and commercial paper
Financial Assets:
Total financial assets	254,939	226,047
Corporate notes and commercial paper | Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	254,939	226,047
U.S. government agency securities
Financial Assets:
Total financial assets	131,708	120,169
U.S. government agency securities | Fair Value, Inputs, Level 2
Financial Assets:
Total financial assets	$ 131,708	$ 120,169