CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Retained Earnings	Treasury Stock	Total
Balance at Dec. 31, 2018	$ 288	$ 115,842	$ 441,010	$ (31,237)	$ 525,903
Balance (in shares) at Dec. 31, 2018	28,787			(2,391)
Stock-based compensation expense		1,259			1,259
Restricted stock units vested (in shares)	34
Issuance of common stock under Employee Stock Purchase Plan		609			609
Issuance of common stock under Employee Stock Purchase Plan (in shares)	19
Shares withheld for taxes paid on stock awards		(409)			(409)
Repurchase of common stock for treasury				$ (4,363)	(4,363)
Repurchase of common stock for treasury (in shares)				(132)
Net income			60,145		60,145
Balance at Sep. 30, 2019	$ 288	117,301	501,155	$ (35,600)	583,144
Balance (in shares) at Sep. 30, 2019	28,840			(2,523)
Balance at Jun. 30, 2019	$ 288	117,212	477,405	$ (34,738)	560,167
Balance (in shares) at Jun. 30, 2019	28,818			(2,500)
Stock-based compensation expense		426			426
Restricted stock units vested (in shares)	22
Shares withheld for taxes paid on stock awards		(337)			(337)
Repurchase of common stock for treasury				$ (862)	(862)
Repurchase of common stock for treasury (in shares)				(23)
Net income			23,750		23,750
Balance at Sep. 30, 2019	$ 288	117,301	501,155	$ (35,600)	583,144
Balance (in shares) at Sep. 30, 2019	28,840			(2,523)
Balance at Dec. 31, 2019	$ 288	118,045	514,694	$ (35,715)	597,312
Balance (in shares) at Dec. 31, 2019	28,870			(2,526)
Stock-based compensation expense		1,866			1,866
Restricted stock units vested	$ 1				1
Restricted stock units vested (in shares)	33
Issuance of common stock under Employee Stock Purchase Plan		536			536
Issuance of common stock under Employee Stock Purchase Plan (in shares)	12
Shares withheld for taxes paid on stock awards		(685)			(685)
Repurchase of common stock for treasury				$ (10,222)	(10,222)
Repurchase of common stock for treasury (in shares)				(247)
Net income			39,469		39,469
Balance at Sep. 30, 2020	$ 289	119,762	554,163	$ (45,937)	628,277
Balance (in shares) at Sep. 30, 2020	28,915			(2,773)
Balance at Jun. 30, 2020	$ 289	119,628	537,233	$ (45,937)	611,213
Balance (in shares) at Jun. 30, 2020	28,892			(2,773)
Stock-based compensation expense		619			619
Restricted stock units vested (in shares)	23
Shares withheld for taxes paid on stock awards		(485)			(485)
Net income			16,930		16,930
Balance at Sep. 30, 2020	$ 289	$ 119,762	$ 554,163	$ (45,937)	$ 628,277
Balance (in shares) at Sep. 30, 2020	28,915			(2,773)