Fair Value of Financial Instruments - Summary of Fair Value Hierarchy for Financial Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 41,504
Fair value, measurements, recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	41,504	$ 38,294
Fair value, measurements, recurring [Member] | Pre-refunded municipal bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	5,928	13,317
Fair value, measurements, recurring [Member] | Money market securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	3,423	3,556
Fair value, measurements, recurring [Member] | Corporate notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	17,220	19,245
Fair value, measurements, recurring [Member] | Federal agency notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,591
Fair value, measurements, recurring [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,304
Fair value, measurements, recurring [Member] | Certificates of deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	2,038	2,176
Fair value, measurements, recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	3,423	3,556
Fair value, measurements, recurring [Member] | Level 1 [Member] | Money market securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	3,423	3,556
Fair value, measurements, recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	38,081	34,738
Fair value, measurements, recurring [Member] | Level 2 [Member] | Pre-refunded municipal bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	5,928	13,317
Fair value, measurements, recurring [Member] | Level 2 [Member] | Corporate notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	17,220	19,245
Fair value, measurements, recurring [Member] | Level 2 [Member] | Federal agency notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,591
Fair value, measurements, recurring [Member] | Level 2 [Member] | Commercial paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,304
Fair value, measurements, recurring [Member] | Level 2 [Member] | Certificates of deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 2,038	$ 2,176