Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Total, Amortized Costs	$ 27,080	$ 79,667
Unrealized Gains	0	0
Unrealized Losses	0	(364)
Fair Value	27,080	79,303
Deferred Compensation Plan Assets [Member]
Debt Securities, Available-for-sale [Line Items]
Total, Amortized Costs	772	2,685
Fair Value	772	2,685
Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Total, Amortized Costs		76,982
Unrealized Gains		0
Unrealized Losses		(364)
Fair Value		76,618
Money Market Mutual Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Total, Amortized Costs	26,308	5,601
Fair Value	$ 26,308	5,601
US Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Total, Amortized Costs		7,501
Unrealized Losses		(34)
Fair Value		7,467
Corporate Bonds and Notes [Member]
Debt Securities, Available-for-sale [Line Items]
Total, Amortized Costs		63,880
Unrealized Gains		0
Unrealized Losses		(330)
Fair Value		$ 63,550