Short-Term Investments - Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 80,212	$ 104,470
Gross Unrealized Gains	62	50
Gross Unrealized Losses	(22)	(14)
Total short-term investments, Estimated Fair Value	80,252	104,506
Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost		1,603
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Total short-term investments, Estimated Fair Value		1,603
State, municipal and local government agencies bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	40,426	59,009
Gross Unrealized Gains	38	45
Gross Unrealized Losses	(15)	(4)
Total short-term investments, Estimated Fair Value	40,449	59,050
Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	33,483	31,568
Gross Unrealized Gains	20	4
Gross Unrealized Losses	(7)	(10)
Total short-term investments, Estimated Fair Value	33,496	31,562
Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2,299	10,287
Gross Unrealized Gains	0	1
Gross Unrealized Losses	0	0
Total short-term investments, Estimated Fair Value	2,299	10,288
U.S. federal government bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	4,004	2,003
Gross Unrealized Gains	4	0
Gross Unrealized Losses	0	0
Total short-term investments, Estimated Fair Value	$ 4,008	$ 2,003