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	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	$ 100,990	$ 98,823
Unrealized Losses	(236)	(268)
Accrued Interest	280	279
Estimated Fair Value	101,034	98,834
Cash & Cash Equivalents	81,364	75,077	$ 52,436	$ 42,037
Investment Securities	19,670	23,757
Cash [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	35,436	28,630
Estimated Fair Value	35,436	28,630
Cash & Cash Equivalents	35,436	28,630
Fair Value, Inputs, Level 1 [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	48,763	51,789
Unrealized Losses	(72)	(67)
Accrued Interest		13
Estimated Fair Value	48,691	51,735
Cash & Cash Equivalents	45,928	46,447
Investment Securities	2,763	5,288
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	40,000	45,000
Estimated Fair Value	40,000	45,000
Cash & Cash Equivalents	40,000	45,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	5,928	1,447
Estimated Fair Value	5,928	1,447
Cash & Cash Equivalents	5,928	1,447
Fair Value, Inputs, Level 1 [Member] | Government and Agency Securities [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	2,835	5,342
Unrealized Losses	(72)	(67)
Accrued Interest		13
Estimated Fair Value	2,763	5,288
Investment Securities	2,763	5,288
Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Cash and Cash Equivalents and Marketable Securities [Line Items]
Cost	16,791	18,404
Unrealized Losses	(164)	(201)
Accrued Interest	280	266
Estimated Fair Value	16,907	18,469
Investment Securities	$ 16,907	$ 18,469