CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Cost of Common Stock in Treasury	Accumulated Other Comprehensive Income (Loss), Net
Balance at Dec. 31, 2016	$ 672,650	$ 436,538	$ 290,824	$ (56,056)	$ 1,344
Cumulative-effect adjustment at Dec. 31, 2016	(65)		(65)
Balance, adjusted at Dec. 31, 2016	672,585	436,538	290,759	(56,056)	1,344
Increase (Decrease) in Shareholders' Equity
Net income	32,875		32,875
Other comprehensive loss	1,093				1,093
Issuance of 44,102 and 60,459 common shares under stock based compensation awards for the six months ended on June 30, 2018 and 2017, respectively	2,305		870	1,435
Cost of 15,789 and 900 shares of common stock acquired for treasury for the six months ended on June 30, 2018 and 2017, respectively	(41)			(41)
Common stock dividend ($0.46 and $0.37 per share for the six months ended on June 30, 2018 and 2017, respectively)	(9,615)		(9,615)
Balance at Jun. 30, 2017	699,202	436,538	314,889	(54,662)	2,437
Balance at Dec. 31, 2017	718,537	436,538	339,959	(54,628)	(3,332)
Cumulative-effect adjustment at Dec. 31, 2017	0		718		(718)
Balance, adjusted at Dec. 31, 2017	718,537	436,538	340,677	(54,628)	(4,050)
Increase (Decrease) in Shareholders' Equity
Net income	41,080		41,080
Other comprehensive loss	(8,365)				(8,365)
Issuance of 44,102 and 60,459 common shares under stock based compensation awards for the six months ended on June 30, 2018 and 2017, respectively	1,770		723	1,047
Cost of 15,789 and 900 shares of common stock acquired for treasury for the six months ended on June 30, 2018 and 2017, respectively	(786)			(786)
Common stock dividend ($0.46 and $0.37 per share for the six months ended on June 30, 2018 and 2017, respectively)	(11,959)		(11,959)
Balance at Jun. 30, 2018	$ 740,277	$ 436,538	$ 370,521	$ (54,367)	$ (12,415)