CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings	Total
BALANCE at Dec. 31, 2016	$ 411	$ (57,387)	$ 309,625	$ 123,984	$ 376,633
BALANCE (in shares) at Dec. 31, 2016	41,123,365
BALANCE (in shares) at Dec. 31, 2016		(3,914,251)
Increase (Decrease) in Stockholders' Equity
Net income				55,272	55,272
Issuance of Stock:
Issuance of shares for options exercised		$ 2,257	(205)		2,052
Issuance of shares for options exercised (in shares)		145,746
Issuance of restricted stock & performance stock		$ 2,037	(421)		1,616
Issuance of restricted stock & performance stock (in shares)		134,646
Shares received in lieu of tax withholding payment on vested restricted stock		$ (1,419)			(1,419)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)		(39,335)
Stock-based compensation			3,785		3,785
Dividends				(10,987)	(10,987)
Share repurchase		$ (9,007)			(9,007)
Minimum number of shares of common stock repurchased		(263,097)
BALANCE at Dec. 31, 2017	$ 411	$ (63,519)	312,784	168,269	$ 417,945
BALANCE (in shares) at Dec. 31, 2017	41,123,365				41,123,365
BALANCE (in shares) at Dec. 31, 2017		(3,936,291)			3,936,291
Increase (Decrease) in Stockholders' Equity
Net income				49,206	$ 49,206
Issuance of Stock:
Issuance of shares for options exercised		$ 2,276	(117)		2,159
Issuance of shares for options exercised (in shares)		131,740
Issuance of restricted stock & performance stock		$ 2,227	(4)		2,223
Issuance of restricted stock & performance stock (in shares)		129,569
Shares received in lieu of tax withholding payment on vested restricted stock		$ (1,540)			(1,540)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)		(36,967)
Stock-based compensation			3,572		3,572
Dividends				(5,760)	(5,760)
Share repurchase		$ (6,830)			(6,830)
Minimum number of shares of common stock repurchased		(165,807)
BALANCE at Jun. 30, 2018	$ 411	$ (67,386)	$ 316,235	$ 211,715	$ 460,975
BALANCE (in shares) at Jun. 30, 2018	41,123,365				41,123,365
BALANCE (in shares) at Jun. 30, 2018		(3,877,756)			3,877,756