Fair Value Measurements - Fair Value of Financial Assets and Liabilities (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Dec. 31, 2013
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Total assets measured at fair value $ 184,644 $ 184,411    
Cash and cash equivalents 159,885 146,569 143,702 123,911
Money market funds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 5,775 11,903 [1]    
Corporate bonds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 112,106 99,474 [2]    
Municipal bonds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 15,246 [3] 18,137 [2]    
Cash and cash equivalents 176      
Commercial paper        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 5,794 9,196    
Assets measured at fair value 51,019 [4] 54,399 [5]    
Cash and cash equivalents 45,225 45,203    
Certificates of deposit        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 498 [2] 498 [2]    
Fair Value, Inputs, Level 1        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Total assets measured at fair value 5,775 11,903    
Fair Value, Inputs, Level 1 | Money market funds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 5,775 [1] 11,903 [1]    
Fair Value, Inputs, Level 1 | Municipal bonds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 0 [3]      
Fair Value, Inputs, Level 1 | Commercial paper        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 0 [4]      
Fair Value, Inputs, Level 1 | Certificates of deposit        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 0 [2]      
Fair Value, Inputs, Level 2        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Total assets measured at fair value 178,869 172,508    
Fair Value, Inputs, Level 2 | Corporate bonds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 112,106 [2] 99,474 [2]    
Fair Value, Inputs, Level 2 | Municipal bonds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 15,246 [3] 18,137 [2]    
Fair Value, Inputs, Level 2 | Commercial paper        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 51,019 [4] 54,399 [5]    
Fair Value, Inputs, Level 2 | Certificates of deposit        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 498 [2] 498 [2]    
Fair Value, Inputs, Level 3        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Total assets measured at fair value 0 0    
Fair Value, Inputs, Level 3 | Municipal bonds        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 0 [3]      
Fair Value, Inputs, Level 3 | Commercial paper        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value 0 [4]      
Fair Value, Inputs, Level 3 | Certificates of deposit        
Fair Values Of Financial Assets And Liabilities Including Derivative Financial Instruments [Line Items]        
Assets measured at fair value $ 0 [2]      
[1] Classified in cash and cash equivalents
[2] Classified in marketable securities
[3] Includes a municipal bond with an original maturity date of three months or less at time purchase of $176 classified in cash and cash equivalents.
[4] Includes commercial paper with maturities of three months or less at time of purchase of $45,225 classified in cash and cash equivalents and commercial paper with maturities of greater than three months of $5,794 classified in marketable securities.
[5] Includes commercial paper with maturities of three months or less at time of purchase of $45,203 classified in cash and cash equivalents and commercial paper with maturities of greater than three months of $9,196 classified in marketable securities.