Condensed Consolidated Statements of Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2019	$ 634,036	$ 106,360	$ 527,746	$ (70)
Balance, Shares at Dec. 31, 2019		24,296,000
Net income	20,159		20,159
Share-based compensation	1,723	$ 1,723
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		50,000
Repurchased common stock	(7,852)	$ (725)	(7,127)
Repurchased common stock, Shares		(164,000)
Taxes paid related to net share settlement of stock awards	(401)	$ (401)
Dividends accrued of $0.435, $0.435, $0.435,$0.42, $0.42 and $0.42 per share for three months ended March 31, 2021, June 30, 2021, September 30, 2021 and three months ended March 31,2020, June 30, 2020, September 30, 2020	(10,219)		(10,219)
Other comprehensive income (loss)	158			158
Balance at Mar. 31, 2020	637,604	$ 106,957	530,559	88
Balance, Shares at Mar. 31, 2020		24,182,000
Balance at Dec. 31, 2019	634,036	$ 106,360	527,746	(70)
Balance, Shares at Dec. 31, 2019		24,296,000
Net income	70,809
Balance at Sep. 30, 2020	661,614	$ 106,079	555,555	(20)
Balance, Shares at Sep. 30, 2020		24,111,000
Balance at Mar. 31, 2020	637,604	$ 106,957	530,559	88
Balance, Shares at Mar. 31, 2020		24,182,000
Net income	22,549		22,549
Share-based compensation	1,501	$ 1,501
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		19,000
Repurchased common stock	(5,648)	$ (509)	(5,139)
Repurchased common stock, Shares		(116,000)
Taxes paid related to net share settlement of stock awards	(1,939)	$ (1,939)
Dividends accrued of $0.435, $0.435, $0.435,$0.42, $0.42 and $0.42 per share for three months ended March 31, 2021, June 30, 2021, September 30, 2021 and three months ended March 31,2020, June 30, 2020, September 30, 2020	(10,194)		(10,194)
Other comprehensive income (loss)	(37)			(37)
Balance at Jun. 30, 2020	643,836	$ 106,010	537,775	51
Balance, Shares at Jun. 30, 2020		24,085,000
Net income	28,101		28,101
Share-based compensation	1,670	$ 1,670
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		28,000
Repurchased common stock	(117)	$ (11)	(106)
Repurchased common stock, Shares		(2,000)
Taxes paid related to net share settlement of stock awards	(1,590)	$ (1,590)
Dividends accrued of $0.435, $0.435, $0.435,$0.42, $0.42 and $0.42 per share for three months ended March 31, 2021, June 30, 2021, September 30, 2021 and three months ended March 31,2020, June 30, 2020, September 30, 2020	(10,215)		(10,215)
Other comprehensive income (loss)	(71)			(71)
Balance at Sep. 30, 2020	661,614	$ 106,079	555,555	(20)
Balance, Shares at Sep. 30, 2020		24,111,000
Balance at Dec. 31, 2020	$ 682,604	$ 106,289	576,419	(104)
Balance, Shares at Dec. 31, 2020	24,128,000	24,128,000
Net income	$ 17,398		17,398
Share-based compensation	1,777	$ 1,777
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		78,000
Taxes paid related to net share settlement of stock awards	(3,482)	$ (3,482)
Dividends accrued of $0.435, $0.435, $0.435,$0.42, $0.42 and $0.42 per share for three months ended March 31, 2021, June 30, 2021, September 30, 2021 and three months ended March 31,2020, June 30, 2020, September 30, 2020	(10,650)		(10,650)
Other comprehensive income (loss)	38			38
Balance at Mar. 31, 2021	687,685	$ 104,584	583,167	(66)
Balance, Shares at Mar. 31, 2021		24,206,000
Balance at Dec. 31, 2020	$ 682,604	$ 106,289	576,419	(104)
Balance, Shares at Dec. 31, 2020	24,128,000	24,128,000
Net income	$ 61,258
Balance at Sep. 30, 2021	$ 712,456	$ 106,744	605,754	(42)
Balance, Shares at Sep. 30, 2021	24,247,000	24,247,000
Balance at Mar. 31, 2021	$ 687,685	$ 104,584	583,167	(66)
Balance, Shares at Mar. 31, 2021		24,206,000
Net income	20,608		20,608
Share-based compensation	1,820	$ 1,820
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		39,000
Taxes paid related to net share settlement of stock awards	(1,346)	$ (1,346)
Dividends accrued of $0.435, $0.435, $0.435,$0.42, $0.42 and $0.42 per share for three months ended March 31, 2021, June 30, 2021, September 30, 2021 and three months ended March 31,2020, June 30, 2020, September 30, 2020	(10,643)		(10,643)
Other comprehensive income (loss)	(3)			(3)
Balance at Jun. 30, 2021	698,121	$ 105,058	593,132	(69)
Balance, Shares at Jun. 30, 2021		24,245,000
Net income	23,252		23,252
Share-based compensation	1,705	$ 1,705
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		2,000
Taxes paid related to net share settlement of stock awards	(19)	$ (19)
Dividends accrued of $0.435, $0.435, $0.435,$0.42, $0.42 and $0.42 per share for three months ended March 31, 2021, June 30, 2021, September 30, 2021 and three months ended March 31,2020, June 30, 2020, September 30, 2020	(10,630)		(10,630)
Other comprehensive income (loss)	27			27
Balance at Sep. 30, 2021	$ 712,456	$ 106,744	$ 605,754	$ (42)
Balance, Shares at Sep. 30, 2021	24,247,000	24,247,000