Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value Based on Three-Tier Fair Value Hierarchy (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	$ 166,392	$ 133,358
Total liabilities measured and recorded at fair value	143	159
Cash equivalents [Member] | Money market funds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	54,923	62,131
Cash equivalents [Member] | Corporate bonds [Member] | Maturity less than 90 days [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	3,614
Cash equivalents [Member] | U.S. federal government bonds [Member] | Maturity less than 90 days [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	3,005
Short-term investments [Member] | Corporate bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	31,562	18,604
Short-term investments [Member] | U.S. federal government bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	2,003	9,230
Short-term investments [Member] | Certificates of deposit [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	1,603
Short-term investments [Member] | State, municipal and local government agencies bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	59,050	38,825
Short-term investments [Member] | Commercial paper [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	10,288	4,195
Prepaids and other current assets [Member] | Foreign exchange forward contracts [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	344	373
Accrued liabilities [Member] | Foreign exchange forward contracts [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total liabilities measured and recorded at fair value	143	159
Level 1 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	59,931	71,361
Level 1 [Member] | Cash equivalents [Member] | Money market funds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	54,923	62,131
Level 1 [Member] | Cash equivalents [Member] | U.S. federal government bonds [Member] | Maturity less than 90 days [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	3,005
Level 1 [Member] | Short-term investments [Member] | U.S. federal government bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	2,003	9,230
Level 2 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	106,461	61,997
Total liabilities measured and recorded at fair value	143	159
Level 2 [Member] | Cash equivalents [Member] | Corporate bonds [Member] | Maturity less than 90 days [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	3,614
Level 2 [Member] | Short-term investments [Member] | Corporate bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	31,562	18,604
Level 2 [Member] | Short-term investments [Member] | Certificates of deposit [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	1,603
Level 2 [Member] | Short-term investments [Member] | State, municipal and local government agencies bonds [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	59,050	38,825
Level 2 [Member] | Short-term investments [Member] | Commercial paper [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	10,288	4,195
Level 2 [Member] | Prepaids and other current assets [Member] | Foreign exchange forward contracts [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total assets measured and recorded at fair value	344	373
Level 2 [Member] | Accrued liabilities [Member] | Foreign exchange forward contracts [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Total liabilities measured and recorded at fair value	$ 143	$ 159