Fair Value Measurements - Summary of Valuation of Investments and Financial Instruments Pricing Levels (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 73,216	$ 79,303
Investment Type [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		76,618
Investment Type [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	72,393	5,601
Investment Type [Member] | US Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		7,467
Investment Type [Member] | Corporate Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		63,550
Deferred Compensation Plan Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	823	2,685
(Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	73,216	8,286
(Level 1) [Member] | Investment Type [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		5,601
(Level 1) [Member] | Investment Type [Member] | Money Market Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	72,393	5,601
(Level 1) [Member] | Deferred Compensation Plan Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	823	2,685
(Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	$ 0	71,017
(Level 2) [Member] | Investment Type [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		71,017
(Level 2) [Member] | Investment Type [Member] | US Government Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		7,467
(Level 2) [Member] | Investment Type [Member] | Corporate Bonds and Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total		$ 63,550