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	64,477			64,477
Other comprehensive income (loss)	(10,035)					(10,035)
Exercise of stock options	4,599	$ 21	4,578
Exercise of stock options (in shares)		206,169
Vested restricted stock			(2)		2
Issuance of / vested performance shares			(1,278)		1,278
Stock option expense	2,188		2,188
Restricted stock expense	183		183
Performance share expense (benefit)	832		832
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
Balance at Mar. 31, 2018	592,737	$ 3,438	170,340	470,103	(12,140)	(39,004)
Balance (in shares) at Mar. 31, 2018		34,383,644
Net income	40,419			40,419
Other comprehensive income (loss)	(13,259)					(13,259)
Exercise of stock options	1,877	$ 10	1,867
Exercise of stock options (in shares)		93,500
Vested restricted stock			(2)		2
Issuance of / vested performance shares			(1,278)		1,278
Stock option expense	1,149		1,149
Restricted stock expense	177		177
Performance share expense (benefit)	227		227
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
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
Balance at Mar. 31, 2019	$ 659,327	$ 3,492	184,410	548,872	(10,777)	(66,670)
Balance (in shares) at Mar. 31, 2019		34,920,124
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	$ 26,027			26,027
Other comprehensive income (loss)	(6,561)					(6,561)
Exercise of stock options	178		178
Exercise of stock options (in shares)		8,375
Issuance of / vested performance shares		$ 5	(5)
Issuance of performance shares (in shares)		46,000
Stock option expense	2,317		2,317
Restricted stock expense	450		450
Performance share expense (benefit)	447		447
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	34,924,499
Balance at Mar. 31, 2019	$ 659,327	$ 3,492	184,410	548,872	(10,777)	(66,670)
Balance (in shares) at Mar. 31, 2019		34,920,124
Net income	15,618			15,618
Other comprehensive income (loss)	(28)					(28)
Exercise of stock options	113		113
Exercise of stock options (in shares)		4,375
Stock option expense	1,255		1,255
Restricted stock expense	354		354
Performance share expense (benefit)	(54)		(54)
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	34,924,499