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	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost	$ 169,665	$ 92,416
Unrealized Gains	111	36
Accrued Interest	143	226
Estimated Fair Value	169,919	92,678
Cash & Cash Equivalents	156,486	76,974
Investment Securities	13,433	15,704
Cash [Member]
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost	155,210	36,593
Estimated Fair Value	155,210	36,593
Cash & Cash Equivalents	155,210	36,593
Fair Value, Inputs, Level 1 [Member]
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost	4,105	43,213
Unrealized Gains	55	2
Accrued Interest	12
Estimated Fair Value	4,172	43,215
Cash & Cash Equivalents	1,276	40,381
Investment Securities	2,896	2,834
Fair Value, Inputs, Level 1 [Member] | Certificates of Deposit [Member]
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost		35,000
Estimated Fair Value		35,000
Cash & Cash Equivalents		35,000
Fair Value, Inputs, Level 1 [Member] | Money Market Funds [Member]
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost	1,276	5,381
Estimated Fair Value	1,276	5,381
Cash & Cash Equivalents	1,276	5,381
Fair Value, Inputs, Level 1 [Member] | Government and Agency Securities [Member]
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost	2,829	2,832
Unrealized Gains	55	2
Accrued Interest	12
Estimated Fair Value	2,896	2,834
Investment Securities	2,896	2,834
Fair Value, Inputs, Level 2 [Member] | Municipal Bonds [Member]
Cash And Cash Equivalents And Marketable securities [Line Items]
Cost	10,350	12,610
Unrealized Gains	56	34
Accrued Interest	131	226
Estimated Fair Value	10,537	12,870
Investment Securities	$ 10,537	$ 12,870