Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Beginning balace at Dec. 31, 2016	$ 338	$ 513,531	$ 331,311	$ (2,381)	$ 842,799
Beginning balace (in shares) at Dec. 31, 2016	34,023,850
Net Income			27,017		27,017
Other comprehensive gain, net of tax				1,100	1,100
Stock-based compensation expense		$ 1,856			$ 1,856
Issuance of common stock related to options exercised, net of shares withheld for payroll taxes (in shares)	2,675	66			66
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes	$ 1	$ (2)			$ (1)
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes (in shares)	(11,788)
Time based stock awards granted (in shares)	91,097
Issuance of common stock related to employee stock purchase plan		205			205
Issuance of common stock related to employee stock purchase plan (in shares)	4,222
Ending balance at Mar. 31, 2017	$ 339	515,656	358,328	(1,281)	873,042
Ending balance (in shares) at Mar. 31, 2017	34,110,056
Beginning balace at Dec. 31, 2017	$ 340	520,304	431,544	(1,750)	$ 950,438
Beginning balace (in shares) at Dec. 31, 2017	34,185,163				34,185,163
Net Income			35,715		$ 35,715
Other comprehensive gain, net of tax				(2,986)	(2,986)
Stock-based compensation expense		1,476			1,476
Issuance of common stock related to options exercised, net of shares withheld for payroll taxes		338			338
Issuance of common stock related to options exercised, net of shares withheld for payroll taxes (in shares)	32,230
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes	$ 1	(1)
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes (in shares)	(12,106)
Time based stock awards granted (in shares)	94,344
Issuance of common stock related to employee stock purchase plan		199			199
Issuance of common stock related to employee stock purchase plan (in shares)	3,425
Reclassification of the Income Tax Effects of the Tax Cuts and Jobs Act from AOCI (ASU 2018-02)			674	(674)
Ending balance at Mar. 31, 2018	$ 341	$ 522,316	$ 467,933	$ (5,410)	$ 985,180
Ending balance (in shares) at Mar. 31, 2018	34,303,056				34,303,056