Financial Instruments (Cash and Available for Sale Securities Adjusted Cost, Gross Unrealized Gains, Gross Unrealized Losses, and Fair Value by Significant Investment Category) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	$ 103,321	$ 98,823
Unrealized Losses	(302)	(268)
Accrued Interest	302	279
Estimated Fair Value	103,321	98,834
Cash & Cash Equivalents	81,076	75,077	$ 63,532	$ 52,436
Investment Securities	22,245	23,757
Cash [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	27,970	28,630
Estimated Fair Value	27,970	28,630
Cash & Cash Equivalents	27,970	28,630
Fair Value, Inputs, Level 1 [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	57,241	51,789
Unrealized Losses	(106)	(67)
Accrued Interest	2	13
Estimated Fair Value	57,137	51,735
Cash & Cash Equivalents	53,106	46,447
Investment Securities	4,031	5,288
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	50,000	45,000
Estimated Fair Value	50,000	45,000
Cash & Cash Equivalents	50,000	45,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	3,106	1,447
Estimated Fair Value	3,106	1,447
Cash & Cash Equivalents	3,106	1,447
Fair Value, Inputs, Level 1 [Member] | Government and Agency Securities [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	4,135	5,342
Unrealized Losses	(106)	(67)
Accrued Interest	2	13
Estimated Fair Value	4,031	5,288
Investment Securities	4,031	5,288
Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	18,110	18,404
Unrealized Losses	(196)	(201)
Accrued Interest	300	266
Estimated Fair Value	18,214	18,469
Investment Securities	$ 18,214	$ 18,469