Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($)	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	AOCI Attributable to Parent [Member]	Total
Balance at Dec. 31, 2017	$ 315,000	$ 217,906,000	$ 381,048,000	$ (57,675,000)	$ (8,986,000)	$ 532,608,000
Net income	0	0	11,412,000	0	0	11,412,000
Award of common shares released from Employee Benefit Trust	0	2,488,000	0	0	0	2,488,000
Vesting of restricted stock unit awards	0	(4,731,000)	(170,000)	4,901,000	0	0
Stock-based compensation expense	0	3,452,000	0	0	0	3,452,000
Repurchase of shares to satisfy tax obligation	0	0	0	(2,050,000)	0	(2,050,000)
Dividends on common stock	0	0	(5,795,000)	0	0	(5,795,000)
Other comprehensive income (loss)	0	0	0	0	(895,000)	(895,000)
Reclassification of the Income tax effects of Tax Cuts and Jobs Act from AOCI to Retained Earnings	0	0	2,073,000	0	(2,073,000)	0
Impact of adoption of Accounting Standard Update 2016-01	0	0	(775,000)	0	775,000	0
Purchase of treasury shares	0	0	0	(5,913,000)	0	(5,913,000)
Balance at Mar. 31, 2018	315,000	219,115,000	387,793,000	(60,737,000)	(11,179,000)	535,307,000
Balance at Dec. 31, 2017	315,000	217,906,000	381,048,000	(57,675,000)	(8,986,000)	532,608,000
Net income						25,335,000
Other comprehensive income (loss)					(1,727,000)	(1,727,000)
Reclassification of the Income tax effects of Tax Cuts and Jobs Act from AOCI to Retained Earnings			2,073,000		(2,073,000)
Impact of adoption of Accounting Standard Update 2016-01					779,000
Balance at Jun. 30, 2018	315,000	220,432,000	395,960,000	(66,656,000)	(12,007,000)	538,044,000
Balance at Mar. 31, 2018	315,000	219,115,000	387,793,000	(60,737,000)	(11,179,000)	535,307,000
Net income	0	0	13,923,000	0	0	13,923,000
Award of common shares released from Employee Benefit Trust	0	90,000	0	0	0	90,000
Exercise of stock options	0	(1,000)	0	7,000	0	6,000
Stock-based compensation expense	0	1,228,000	0	0	0	1,228,000
Repurchase of shares to satisfy tax obligation	0	0	0	(1,000)	0	(1,000)
Dividends on common stock	0	0	(5,752,000)	0	0	(5,752,000)
Other comprehensive income (loss)	0	0	0	0	(832,000)	(832,000)
Impact of adoption of Accounting Standard Update 2016-01	0	0	(4,000)	0	4,000	0
Purchase of treasury shares	0	0	0	(5,925,000)	0	(5,925,000)
Balance at Jun. 30, 2018	315,000	220,432,000	395,960,000	(66,656,000)	(12,007,000)	538,044,000
Balance at Dec. 31, 2018	315,000	222,720,000	414,327,000	(75,146,000)	(12,752,000)	549,464,000
Impact of adoption of ASC 842 - Leases | Accounting Standards Update 2016-02 [Member]	0	0	2,716,000	0	0	2,716,000
Net income	0	0	7,068,000	0	0	7,068,000
Award of common shares released from Employee Benefit Trust	0	2,086,000	0	0	0	2,086,000
Vesting of restricted stock unit awards	0	(5,878,000)	(210,000)	6,088,000	0	0
Exercise of stock options	0	0	(3,000)	6,000	0	3,000
Stock-based compensation expense	0	3,931,000	0	0	0	3,931,000
Repurchase of shares to satisfy tax obligation	0	0	0	(1,877,000)	0	(1,877,000)
Dividends on common stock	0	0	(6,042,000)	0	0	(6,042,000)
Other comprehensive income (loss)	0	0	0	0	2,210,000	2,210,000
Balance at Mar. 31, 2019	315,000	222,859,000	417,856,000	(70,929,000)	(10,542,000)	559,559,000
Balance at Dec. 31, 2018	315,000	222,720,000	414,327,000	(75,146,000)	(12,752,000)	549,464,000
Net income						17,624,000
Other comprehensive income (loss)					2,136,000	2,136,000
Reclassification of the Income tax effects of Tax Cuts and Jobs Act from AOCI to Retained Earnings			0
Balance at Jun. 30, 2019	315,000	224,231,000	422,373,000	(70,913,000)	(10,616,000)	565,390,000
Balance at Mar. 31, 2019	315,000	222,859,000	417,856,000	(70,929,000)	(10,542,000)	559,559,000
Net income	0	0	10,556,000	0	0	10,556,000
Award of common shares released from Employee Benefit Trust	0	81,000	0	0	0	81,000
Vesting of restricted stock unit awards	0	(24,000)	0	24,000	0	0
Stock-based compensation expense	0	1,315,000	0	0	0	1,315,000
Repurchase of shares to satisfy tax obligation	0	0	0	(8,000)	0	(8,000)
Dividends on common stock	0	0	(6,039,000)	0	0	(6,039,000)
Other comprehensive income (loss)	0	0	0	0	(74,000)	(74,000)
Balance at Jun. 30, 2019	$ 315,000	$ 224,231,000	$ 422,373,000	$ (70,913,000)	$ (10,616,000)	$ 565,390,000