Condensed Consolidated Statement Of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
Beginning balance at Dec. 31, 2017	$ 652,745	$ 277	$ 203,990	$ 475,506	$ 2,439	$ (29,467)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,336			47,336
Issuance of shares of common stock pursuant to stock-based awards, net of shares tendered for payment of taxes	(14,085)	2	(14,087)
Stock-based compensation expense	5,543		5,543
Other comprehensive income (loss)					1,110
Cash dividends	(13,858)			(13,858)
Dividend equivalents on stock-based awards	0		319	(319)
Ending balance at Mar. 31, 2018	678,791	279	195,765	508,665	3,549	(29,467)
Beginning balance at Dec. 31, 2017	652,745	277	203,990	475,506	2,439	(29,467)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	128,372
Ending balance at Sep. 30, 2018	735,646	279	204,748	558,742	1,344	(29,467)
Beginning balance at Mar. 31, 2018	678,791	279	195,765	508,665	3,549	(29,467)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,224			47,224
Issuance of shares of common stock pursuant to stock-based awards, net of shares tendered for payment of taxes	(29)		(29)
Stock-based compensation expense	4,219		4,219
Other comprehensive income (loss)	(1,899)				(1,899)
Cash dividends	(15,127)			(15,127)
Dividend equivalents on stock-based awards	0		351	(351)
Ending balance at Jun. 30, 2018	713,179	279	200,306	540,411	1,650	(29,467)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	33,812			33,812
Issuance of shares of common stock pursuant to stock-based awards, net of shares tendered for payment of taxes	0		0
Stock-based compensation expense	4,090		4,090
Other comprehensive income (loss)	(306)				(306)
Cash dividends	(15,129)			(15,129)
Dividend equivalents on stock-based awards	0		352	(352)
Ending balance at Sep. 30, 2018	735,646	279	204,748	558,742	1,344	(29,467)
Beginning balance at Dec. 31, 2018	706,255	280	203,246	563,496	(2,605)	(58,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	34,366			34,366
Issuance of shares of common stock pursuant to stock-based awards, net of shares tendered for payment of taxes	(6,348)	1	(6,349)
Stock-based compensation expense	3,733		3,733
Other comprehensive income (loss)	(1,328)				(1,328)
Cash dividends	(14,999)			(14,999)
Dividend equivalents on stock-based awards	0		304	(304)
Ending balance at Mar. 31, 2019	721,679	281	200,934	582,559	(3,933)	(58,162)
Beginning balance at Dec. 31, 2018	706,255	280	203,246	563,496	(2,605)	(58,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	117,702
Ending balance at Sep. 30, 2019	777,381	281	209,053	632,725	(6,516)	(58,162)
Beginning balance at Mar. 31, 2019	721,679	281	200,934	582,559	(3,933)	(58,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,527			47,527
Issuance of shares of common stock pursuant to stock-based awards, net of shares tendered for payment of taxes	(795)		(795)
Stock-based compensation expense	4,115		4,115
Other comprehensive income (loss)	(3,383)				(3,383)
Cash dividends	(16,267)			(16,267)
Dividend equivalents on stock-based awards	0		318	(318)
Ending balance at Jun. 30, 2019	752,876	281	204,572	613,501	(7,316)	(58,162)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	35,809
Issuance of shares of common stock pursuant to stock-based awards, net of shares tendered for payment of taxes	(50)		(50)
Stock-based compensation expense	4,213		4,213
Other comprehensive income (loss)	800				800
Cash dividends	(16,267)			(16,267)
Dividend equivalents on stock-based awards	0		318	(318)
Ending balance at Sep. 30, 2019	$ 777,381	$ 281	$ 209,053	$ 632,725	$ (6,516)	$ (58,162)