Short-Term Investments - Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 31,845	$ 80,212
Gross Unrealized Gains	19	62
Gross Unrealized Losses	(17)	(22)
Total short-term investments, Estimated Fair Value	31,847	80,252
State, municipal and local government agencies bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	13,946	40,426
Gross Unrealized Gains	16	38
Gross Unrealized Losses	(1)	(15)
Total short-term investments, Estimated Fair Value	13,961	40,449
Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	17,899	33,483
Gross Unrealized Gains	3	20
Gross Unrealized Losses	(16)	(7)
Total short-term investments, Estimated Fair Value	17,886	33,496
Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		2,299
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Total short-term investments, Estimated Fair Value		2,299
U.S. federal government bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		4,004
Gross Unrealized Gains		4
Gross Unrealized Losses		0
Total short-term investments, Estimated Fair Value		$ 4,008