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] 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, 2016									$ 315	$ 214,462	$ 361,192	$ (53,754)	$ (8,362)	$ 513,853
Reclassification of the Income Tax Effects of the Tax Cuts and Jobs Act from Accumulated Other Comprehensive Income (Loss) to Retained Earnings
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 loss													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 loss													(88)	(88)
Balance at Sep. 30, 2017									315	216,929	380,316	(51,287)	(6,664)	539,609
Balance at Dec. 31, 2017									315	217,906	381,048	(57,675)	(8,986)	532,608
Reclassification of the Income Tax Effects of the Tax Cuts and Jobs Act from Accumulated Other Comprehensive Income (Loss) to Retained Earnings											2,073		(2,073)	2,073
Impact of adoption of Accounting Standard Update 2016-01											(779)		779
Net income											42,668			42,668
Award of common shares released from Employee Benefit Trust										2,652				2,652
Vesting of restricted stock unit awards										(4,908)	(176)	5,084
Exercise of stock options										(1)		7		6
Stock-based compensation expense										5,973				5,973
Purchase of treasury shares				$ (19,500)		$ (19,500)	$ (2,138)	$ (2,138)
Dividends on common stock											(17,244)			(17,244)
Other comprehensive loss													(3,269)	(3,269)
Balance at Sep. 30, 2018									315	221,622	407,590	(74,222)	(13,549)	541,756
Balance at Jun. 30, 2018													(12,007)
Net income														17,333
Other comprehensive loss													(1,542)	(1,542)
Balance at Sep. 30, 2018									$ 315	$ 221,622	$ 407,590	$ (74,222)	$ (13,549)	$ 541,756