Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Shares Purchased to Fund Options Exercised [Member] Common Stock [Member]	Shares Purchased to Fund Options Exercised [Member] Additional Paid-in Capital [Member]	Shares Purchased to Fund Options Exercised [Member] Retained Earnings [Member]	Shares Purchased to Fund Options Exercised [Member] Treasury Stock [Member]	Shares Purchased to Fund Options Exercised [Member] AOCI Attributable to Parent [Member]	Shares Purchased to Fund Options Exercised [Member]	Restricted Stock Awards Repurchased to Satisfy Tax Obligations [Member] Retained Earnings [Member]	Restricted Stock Awards Repurchased to Satisfy Tax Obligations [Member] Treasury Stock [Member]	Restricted Stock Awards Repurchased to Satisfy Tax Obligations [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	AOCI Attributable to Parent [Member]	Total
Balance at Dec. 31, 2015										$ 315	$ 210,652	$ 316,530	$ (48,868)	$ (5,562)	$ 473,067
Net Income												50,630			50,630
Award of common shares released from Employee Benefit Trust											1,984				1,984
Vesting of restricted stock unit awards											(4,049)	(397)	4,446
Exercise of stock options											15	(34)	151		132
Stock-based compensation expense											4,416				4,416
Purchase of treasury shares				$ (7,492)		$ (7,492)		$ (1,610)	$ (1,610)
Dividends on common stock												(14,787)			(14,787)
Other comprehensive income														5,811	5,811
Balance at Sep. 30, 2016										315	213,488	351,942	(53,373)	249	512,621
Stock-based income tax benefit											470				470
Balance at Jun. 30, 2016														3,088
Net Income															10,634
Other comprehensive income														(2,839)	(2,839)
Balance at Sep. 30, 2016										315	213,488	351,942	(53,373)	249	512,621
Balance at Dec. 31, 2016										315	214,462	361,192	(53,754)	(8,362)	513,853
Net Income												35,164			35,164
Award of common shares released from Employee Benefit Trust											2,433				2,433
Vesting of restricted stock unit awards											(5,052)	(271)	5,323
Exercise of stock options											(6)	(40)	46
Stock-based compensation expense											5,092				5,092
Purchase of treasury shares				$ (278)		$ (278)		$ (2,624)	$ (2,624)
Dividends on common stock												(15,729)			(15,729)
Other comprehensive income														1,698	1,698
Balance at Sep. 30, 2017										315	216,929	380,316	(51,287)	(6,664)	539,609
Balance at Jun. 30, 2017														(6,576)
Net Income															10,179
Other comprehensive income														(88)	(88)
Balance at Sep. 30, 2017										$ 315	$ 216,929	$ 380,316	$ (51,287)	$ (6,664)	$ 539,609