Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]	Accumulated Other Comprehensive Loss [Member]
Balance at Dec. 31, 2017	$ 544,864	$ 3,427	$ 165,979	$ 429,826	$ (12,140)	$ (42,228)
Balance (in shares) at Dec. 31, 2017		34,270,975
Impact of change in accounting policy at Dec. 31, 2017	16,219			16,219
Adjusted balance at Dec. 31, 2017	561,083	$ 3,427	165,979	446,045	(12,140)	(42,228)
Net income	91,206			91,206
Other comprehensive income (loss)	(9,801)					(9,801)
Exercise of stock options	12,684	$ 54	12,630
Exercise of stock options (in shares)		542,974
Vested restricted stock			(2)		2
Issuance of / vested performance shares			(1,278)		1,278
Stock option expense	3,182		3,182
Restricted stock expense	445		445
Performance share expense (benefit)	479		479
Balance at Sep. 30, 2018	659,278	$ 3,481	181,435	537,251	(10,860)	(52,029)
Balance (in shares) at Sep. 30, 2018		34,813,949
Balance at Jun. 30, 2018	623,327	$ 3,448	172,480	510,522	(10,860)	(52,263)
Balance (in shares) at Jun. 30, 2018		34,477,144
Net income	26,729			26,729
Other comprehensive income (loss)	234					234
Exercise of stock options	8,085	$ 33	8,052
Exercise of stock options (in shares)		336,805
Stock option expense	994		994
Restricted stock expense	262		262
Performance share expense (benefit)	(353)		(353)
Balance at Sep. 30, 2018	659,278	$ 3,481	181,435	537,251	(10,860)	(52,029)
Balance (in shares) at Sep. 30, 2018		34,813,949
Balance at Dec. 31, 2018	$ 653,727	$ 3,487	182,691	538,463	(10,777)	(60,137)
Balance (in shares) at Dec. 31, 2018	34,870,124	34,870,124
Net income	$ 35,573			35,573
Other comprehensive income (loss)	(20,123)					(20,123)
Exercise of stock options	1,748	$ 7	1,741
Exercise of stock options (in shares)		72,352
Issuance of / vested performance shares		$ 5	(5)
Issuance of performance shares (in shares)		46,000
Stock option expense	3,611		3,611
Restricted stock expense	909		909
Performance share expense (benefit)	445		445
Balance at Sep. 30, 2019	$ 675,890	$ 3,499	189,392	574,036	(10,777)	(80,260)
Balance (in shares) at Sep. 30, 2019	34,988,476	34,988,476
Balance at Jun. 30, 2019	$ 676,585	$ 3,492	186,078	564,490	(10,777)	(66,698)
Balance (in shares) at Jun. 30, 2019		34,924,499
Net income	9,546			9,546
Other comprehensive income (loss)	(13,562)					(13,562)
Exercise of stock options	1,570	$ 7	1,563
Exercise of stock options (in shares)		63,977
Stock option expense	1,294		1,294
Restricted stock expense	459		459
Performance share expense (benefit)	(2)		(2)
Balance at Sep. 30, 2019	$ 675,890	$ 3,499	$ 189,392	$ 574,036	$ (10,777)	$ (80,260)
Balance (in shares) at Sep. 30, 2019	34,988,476	34,988,476