Short-Term Investments - Summary of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 104,470	$ 70,819
Gross Unrealized Gains	50	56
Gross Unrealized Losses	(14)	(21)
Total short-term investments, Estimated Fair Value	104,506	70,854
Certificates of deposit [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	1,603
Gross Unrealized Gains
Gross Unrealized Losses
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	59,009	38,785
Gross Unrealized Gains	45	46
Gross Unrealized Losses	(4)	(6)
Total short-term investments, Estimated Fair Value	59,050	38,825
Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	31,568	18,613
Gross Unrealized Gains	4	6
Gross Unrealized Losses	(10)	(15)
Total short-term investments, Estimated Fair Value	31,562	18,604
Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	10,287	4,195
Gross Unrealized Gains	1
Gross Unrealized Losses
Total short-term investments, Estimated Fair Value	10,288	4,195
U.S. federal government bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	2,003	9,226
Gross Unrealized Gains		4
Gross Unrealized Losses
Total short-term investments, Estimated Fair Value	$ 2,003	$ 9,230