Investments - Components of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	$ 73,216	$ 79,667
Unrealized Gains	0	0
Unrealized Losses	0	(364)
Fair Value	73,216	79,303
Investment Type [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		76,982
Unrealized Gains		0
Unrealized Losses		(364)
Fair Value		76,618
Investment Type [Member] | Money Market Mutual Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	72,393	5,601
Fair Value	72,393	5,601
Investment Type [Member] | US Government Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		7,501
Unrealized Losses		(34)
Fair Value		7,467
Investment Type [Member] | Corporate Bonds and Notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis		63,880
Unrealized Gains		0
Unrealized Losses		(330)
Fair Value		63,550
Deferred Compensation Plan Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost Basis	823	2,685
Fair Value	$ 823	$ 2,685