Schedule of Marketable Securities (Detail) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		$ 99,724	$ 61,753
Unrealized Gains		22	1
Unrealized Losses		(454)	(285)
Fair Value	[1]	99,292	61,469
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		55,466	32,996
Unrealized Gains		20
Unrealized Losses		(112)	(148)
Fair Value		55,374	32,848
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		44,258	28,757
Unrealized Gains		2	1
Unrealized Losses		(342)	(137)
Fair Value		$ 43,918	$ 28,621

[1] On January 1, 2018 the Company adopted the ASC 606 revenue recognition standard and has adjusted prior periods to conform. See Note 2. "New Accounting Pronouncements" for additional information.