Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Adjustment to adopt credit-loss guidance	[1]	Balance, January 1, 2020	Common Stock	Common Stock Balance, January 1, 2020	Additional Paid-in Capital	Additional Paid-in Capital Balance, January 1, 2020	Accumulated Other Comprehensive Income, net of taxes	Accumulated Other Comprehensive Income, net of taxes Balance, January 1, 2020	Retained Earnings	Retained Earnings Adjustment to adopt credit-loss guidance	[1]	Retained Earnings Balance, January 1, 2020	Treasury Stock	Treasury Stock Balance, January 1, 2020
Balance (in shares) at Dec. 31, 2018					75,344,000
Beginning balance at Dec. 31, 2018	$ 440,780				$ 753		$ 259,440		$ 158		$ 190,092				$ (9,663)
Comprehensive income:
Net income for the period	9,156										9,156
Unrealized loss on available-for-sale marketable securities, net of taxes	1,416								1,416
Total comprehensive income	10,572
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)					115,000
Exercise of stock options and other share-based compensation, net of shares tendered for payment	1,913				$ 2		1,911
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(579)						(579)
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,656						1,656
Treasury shares issued for Deferred Compensation Plan funding and redemptions	536						535								1
Shares issued pursuant to Employee Stock Plan	2,130						1,781								349
Dividends paid and accrued, $0.20 per share	(14,656)										(14,656)
Shares issued pursuant to Dividend Reinvestment Plan	23						18								5
Other (in shares)					6,000
Other	174						174
Balance (in shares) at Mar. 31, 2019					75,465,000
Ending balance at Mar. 31, 2019	442,549				$ 755		264,936		1,574		184,592				(9,308)
Balance (in shares) at Dec. 31, 2018					75,344,000
Beginning balance at Dec. 31, 2018	440,780				$ 753		259,440		158		190,092				(9,663)
Comprehensive income:
Net income for the period	27,342
Unrealized loss on available-for-sale marketable securities, net of taxes	2,207
Total comprehensive income	29,549
Balance (in shares) at Jun. 30, 2019					75,519,000
Ending balance at Jun. 30, 2019	449,648				$ 755		267,853		2,365		188,025				(9,350)
Balance (in shares) at Dec. 31, 2018					75,344,000
Beginning balance at Dec. 31, 2018	440,780				$ 753		259,440		158		190,092				(9,663)
Balance (in shares) at Dec. 31, 2019					75,557,000	75,557,000
Ending balance at Dec. 31, 2019	$ 460,305	$ (32,099)		$ 428,206	$ 756	$ 756	270,614	$ 270,614	2,919	$ 2,919	195,455	$ (32,099)		$ 163,356	(9,439)	$ (9,439)
Comprehensive income:
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Balance (in shares) at Mar. 31, 2019					75,465,000
Beginning balance at Mar. 31, 2019	$ 442,549				$ 755		264,936		1,574		184,592				(9,308)
Comprehensive income:
Net income for the period	18,186										18,186
Unrealized loss on available-for-sale marketable securities, net of taxes	791								791
Total comprehensive income	18,977
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)					54,000
Exercise of stock options and other share-based compensation, net of shares tendered for payment	1,186						1,186
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,668						1,668
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(2)						45								(47)
Dividends paid and accrued, $0.20 per share	(14,753)										(14,753)
Shares issued pursuant to Dividend Reinvestment Plan	23						18								5
Balance (in shares) at Jun. 30, 2019					75,519,000
Ending balance at Jun. 30, 2019	449,648				$ 755		267,853		2,365		188,025				(9,350)
Balance (in shares) at Dec. 31, 2019					75,557,000	75,557,000
Beginning balance at Dec. 31, 2019	460,305	(32,099)		428,206	$ 756	$ 756	270,614	270,614	2,919	2,919	195,455	(32,099)		163,356	(9,439)	(9,439)
Comprehensive income:
Net income for the period	20,190										20,190
Unrealized loss on available-for-sale marketable securities, net of taxes	(533)								(533)
Total comprehensive income	19,657
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)					162,000
Exercise of stock options and other share-based compensation, net of shares tendered for payment	1,724				$ 1		1,723
Payment of statutory withholding on issuance of restricted stock and restricted stock units	(661)						(661)
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,822						1,822
Treasury shares issued for Deferred Compensation Plan funding and redemptions	528						417								111
Shares issued pursuant to Employee Stock Plan	1,835						1,329								506
Dividends paid and accrued, $0.20 per share	(15,142)										(15,142)
Shares issued pursuant to Dividend Reinvestment Plan	23						16								7
Other (in shares)					6,000
Other	187						187
Balance (in shares) at Mar. 31, 2020					75,725,000
Ending balance at Mar. 31, 2020	438,179				$ 757		275,447		2,386		168,404				(8,815)
Balance (in shares) at Dec. 31, 2019					75,557,000	75,557,000
Beginning balance at Dec. 31, 2019	460,305	$ (32,099)		$ 428,206	$ 756	$ 756	270,614	$ 270,614	2,919	$ 2,919	195,455	$ (32,099)		$ 163,356	(9,439)	$ (9,439)
Comprehensive income:
Net income for the period	43,291
Unrealized loss on available-for-sale marketable securities, net of taxes	1,123
Total comprehensive income	44,414
Balance (in shares) at Jun. 30, 2020					75,732,000
Ending balance at Jun. 30, 2020	449,687				$ 757		277,482		4,042		176,268				(8,862)
Balance (in shares) at Mar. 31, 2020					75,725,000
Beginning balance at Mar. 31, 2020	438,179				$ 757		275,447		2,386		168,404				(8,815)
Comprehensive income:
Net income for the period	23,101										23,101
Unrealized loss on available-for-sale marketable securities, net of taxes	1,656								1,656
Total comprehensive income	24,757
Exercise of stock options and other stock-based compensation, net of shares tendered for payment (in shares)					7,000
Exercise of stock options and other share-based compensation, net of shares tendered for payment	138						138
Share-based compensation expense — stock options, restricted stock and restricted stock units	1,830						1,830
Treasury shares issued for Deferred Compensation Plan funding and redemptions	(2)						51								(53)
Dividends paid and accrued, $0.20 per share	(15,237)										(15,237)
Shares issued pursuant to Dividend Reinvestment Plan	22						16								6
Balance (in shares) at Jun. 30, 2020					75,732,000
Ending balance at Jun. 30, 2020	$ 449,687				$ 757		$ 277,482		$ 4,042		$ 176,268				$ (8,862)

[1] See Note 4—Allowance for Doubtful Accounts regarding the new credit-loss guidance