Fair Value Measurements (Detail) - Financial Assets Measured At Fair Value On A Recurring Basis (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Cash equivalents
Cash equivalents	$ 9,609	$ 38,336
Short-term investments
Short-term investments	239,514	190,567
Muncipal bonds
Short-term investments
Short-term investments	120,282	146,100
Commercial paper
Short-term investments
Short-term investments	2,498	31,597
Corporate bonds
Short-term investments
Short-term investments	68,672	7,875
U.S. government and agency securities
Short-term investments
Short-term investments	48,062	4,995
Commercial paper
Cash equivalents
Cash equivalents	5,749	34,498
U.S. government and agency securities
Cash equivalents
Cash equivalents	2,000	3,801
Money market funds
Cash equivalents
Cash equivalents	451	37
Muncipal bonds
Cash equivalents
Cash equivalents	1,409
Fair Value, Inputs, Level 1
Cash equivalents
Cash equivalents	451	37
Short-term investments
Short-term investments	0	0
Fair Value, Inputs, Level 1 | Muncipal bonds
Short-term investments
Short-term investments	0	0
Fair Value, Inputs, Level 1 | Commercial paper
Short-term investments
Short-term investments	0	0
Fair Value, Inputs, Level 1 | Corporate bonds
Short-term investments
Short-term investments	0	0
Fair Value, Inputs, Level 1 | U.S. government and agency securities
Short-term investments
Short-term investments	0	0
Fair Value, Inputs, Level 1 | Commercial paper
Cash equivalents
Cash equivalents	0	0
Fair Value, Inputs, Level 1 | U.S. government and agency securities
Cash equivalents
Cash equivalents	0	0
Fair Value, Inputs, Level 1 | Money market funds
Cash equivalents
Cash equivalents	451	37
Fair Value, Inputs, Level 1 | Muncipal bonds
Cash equivalents
Cash equivalents	0
Fair Value, Inputs, Level 2
Cash equivalents
Cash equivalents	9,158	38,299
Short-term investments
Short-term investments	239,514	190,567
Fair Value, Inputs, Level 2 | Muncipal bonds
Short-term investments
Short-term investments	120,282	146,100
Fair Value, Inputs, Level 2 | Commercial paper
Short-term investments
Short-term investments	2,498	31,597
Fair Value, Inputs, Level 2 | Corporate bonds
Short-term investments
Short-term investments	68,672	7,875
Fair Value, Inputs, Level 2 | U.S. government and agency securities
Short-term investments
Short-term investments	48,062	4,995
Fair Value, Inputs, Level 2 | Commercial paper
Cash equivalents
Cash equivalents	5,749	34,498
Fair Value, Inputs, Level 2 | U.S. government and agency securities
Cash equivalents
Cash equivalents	2,000	3,801
Fair Value, Inputs, Level 2 | Money market funds
Cash equivalents
Cash equivalents	0	0
Fair Value, Inputs, Level 2 | Muncipal bonds
Cash equivalents
Cash equivalents	$ 1,409