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] Treasury Stock [Member]	Shares Purchased to Fund Options Exercised [Member] Retained Earnings [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]	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Total
Balance at Dec. 31, 2016									$ 315	$ 214,462	$ 361,192	$ (53,754)	$ (8,362)	$ 513,853
Net income											24,985			24,985
Award of common shares released from Employee Benefit Trust										2,363				2,363
Vesting of restricted stock unit awards										(4,562)	(262)	4,824
Exercise of stock options										(6)	(40)	46
Stock-based compensation expense										4,190				4,190
Purchase of treasury shares				$ (278)		$ (278)	$ (2,321)	$ (2,321)
Dividends on common stock											(10,487)			(10,487)
Other comprehensive loss													1,786	1,786
Balance at Jun. 30, 2017									315	216,447	375,388	(51,483)	(6,576)	534,091
Balance at Mar. 31, 2017													(7,134)
Net income														12,725
Other comprehensive loss													558	558
Balance at Jun. 30, 2017									315	216,447	375,388	(51,483)	(6,576)	534,091
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)
Impact of adoption of Accounting Standard Update 2016-01											(779)		779
Net income											25,335			25,335
Award of common shares released from Employee Benefit Trust										2,578				2,578
Vesting of restricted stock unit awards										(4,731)	(170)	4,901
Exercise of stock options										(1)		7		6
Stock-based compensation expense										4,680				4,680
Purchase of treasury shares				$ (11,838)		$ (11,838)	$ (2,051)	$ (2,051)
Dividends on common stock											(11,547)			(11,547)
Other comprehensive loss													(1,727)	(1,727)
Balance at Jun. 30, 2018									315	220,432	395,960	(66,656)	(12,007)	538,044
Balance at Mar. 31, 2018													(11,175)
Net income														13,923
Other comprehensive loss													(832)	(832)
Balance at Jun. 30, 2018									$ 315	$ 220,432	$ 395,960	$ (66,656)	$ (12,007)	$ 538,044