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, 2017	$ 524,184	$ 102,947	$ 421,405	$ (168)
Balance, Shares at Dec. 31, 2017		24,052,000
Net income	14,466		14,466
Share-based compensation	864	$ 864
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		50,000
Taxes paid related to net share settlement of stock awards	(971)	$ (971)
Dividends accrued of $0.375, $0.375, $0.375, $0.34, $0.34, and $0.34 per share for three months ended March 31, 2019, June 30, 2019, September 30, 2019 and three months ended March 31 2018, June 30, 2018, September 30, 2018	(8,237)		(8,237)
Other comprehensive income (loss)	(22)			(22)
Balance at Mar. 31, 2018	530,284	$ 102,840	427,634	(190)
Balance, Shares at Mar. 31, 2018		24,102,000
Balance at Dec. 31, 2017	524,184	$ 102,947	421,405	(168)
Balance, Shares at Dec. 31, 2017		24,052,000
Net income	55,157
Balance at Sep. 30, 2018	554,547	$ 102,753	451,794
Balance, Shares at Sep. 30, 2018		24,176,000
Balance at Mar. 31, 2018	530,284	$ 102,840	427,634	(190)
Balance, Shares at Mar. 31, 2018		24,102,000
Net income	15,912		15,912
Share-based compensation	964	$ 964
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		68,000
Taxes paid related to net share settlement of stock awards	(1,821)	$ (1,821)
Dividends accrued of $0.375, $0.375, $0.375, $0.34, $0.34, and $0.34 per share for three months ended March 31, 2019, June 30, 2019, September 30, 2019 and three months ended March 31 2018, June 30, 2018, September 30, 2018	(8,267)		(8,267)
Other comprehensive income (loss)	123			123
Balance at Jun. 30, 2018	537,195	$ 101,983	435,279	(67)
Balance, Shares at Jun. 30, 2018		24,170,000
Net income	24,779		24,779
Share-based compensation	982	$ 982
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		6,000
Taxes paid related to net share settlement of stock awards	(212)	$ (212)
Dividends accrued of $0.375, $0.375, $0.375, $0.34, $0.34, and $0.34 per share for three months ended March 31, 2019, June 30, 2019, September 30, 2019 and three months ended March 31 2018, June 30, 2018, September 30, 2018	(8,264)		(8,264)
Other comprehensive income (loss)	67			67
Balance at Sep. 30, 2018	554,547	$ 102,753	451,794
Balance, Shares at Sep. 30, 2018		24,176,000
Balance at Dec. 31, 2018	$ 571,535	$ 103,801	467,783	(49)
Balance, Shares at Dec. 31, 2018	24,182,000	24,182,000
Net income	$ 18,449		18,449
Share-based compensation	1,392	$ 1,392
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		40,000
Taxes paid related to net share settlement of stock awards	(1,555)	$ (1,555)
Dividends accrued of $0.375, $0.375, $0.375, $0.34, $0.34, and $0.34 per share for three months ended March 31, 2019, June 30, 2019, September 30, 2019 and three months ended March 31 2018, June 30, 2018, September 30, 2018	(9,151)		(9,151)
Other comprehensive income (loss)	(27)			(27)
Balance at Mar. 31, 2019	580,643	$ 103,638	477,081	(76)
Balance, Shares at Mar. 31, 2019		24,222,000
Balance at Dec. 31, 2018	$ 571,535	$ 103,801	467,783	(49)
Balance, Shares at Dec. 31, 2018	24,182,000	24,182,000
Net income	$ 70,405
Balance at Sep. 30, 2019	$ 616,715	$ 106,033	510,702	(20)
Balance, Shares at Sep. 30, 2019	24,285,000	24,285,000
Balance at Mar. 31, 2019	$ 580,643	$ 103,638	477,081	(76)
Balance, Shares at Mar. 31, 2019		24,222,000
Net income	19,488		19,488
Share-based compensation	1,354	$ 1,354
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		31,000
Taxes paid related to net share settlement of stock awards	(8)	$ (8)
Dividends accrued of $0.375, $0.375, $0.375, $0.34, $0.34, and $0.34 per share for three months ended March 31, 2019, June 30, 2019, September 30, 2019 and three months ended March 31 2018, June 30, 2018, September 30, 2018	(9,162)		(9,162)
Other comprehensive income (loss)	(6)			(6)
Balance at Jun. 30, 2019	592,309	$ 104,984	487,407	(82)
Balance, Shares at Jun. 30, 2019		24,253,000
Net income	32,468		32,468
Share-based compensation	1,350	$ 1,350
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		32,000
Taxes paid related to net share settlement of stock awards	(301)	$ (301)
Dividends accrued of $0.375, $0.375, $0.375, $0.34, $0.34, and $0.34 per share for three months ended March 31, 2019, June 30, 2019, September 30, 2019 and three months ended March 31 2018, June 30, 2018, September 30, 2018	(9,173)		(9,173)
Other comprehensive income (loss)	62			62
Balance at Sep. 30, 2019	$ 616,715	$ 106,033	$ 510,702	$ (20)
Balance, Shares at Sep. 30, 2019	24,285,000	24,285,000