Description of Business and Summary of Significant Accounting Policies - Summary of Fair Value Hierarchy for Financial Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 66,676
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	66,676	$ 59,769
Fair Value, Measurements, Recurring [Member] | Pre-refunded Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		6,716
Fair Value, Measurements, Recurring [Member] | Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	2,197	1,218
Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	49,834	16,757
Fair Value, Measurements, Recurring [Member] | Federal Agency Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	10,715	16,810
Fair Value, Measurements, Recurring [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,934	15,753
Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,996	2,515
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	2,197	1,218
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Pre-refunded Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	2,197	1,218
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Federal Agency Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	64,479	58,551
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Pre-refunded Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		6,716
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	49,834	16,757
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Federal Agency Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	10,715	16,810
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,934	15,753
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	1,996	2,515
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Pre-refunded Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Federal Agency Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0	0
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasuries [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 0	$ 0