Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Additional Paid In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2018		34,387,919
Beginning balance at Dec. 31, 2018	$ 1,108,941	$ 342	$ 528,380	$ 584,494	$ (4,275)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	107,487			107,487
Other comprehensive income, net of tax	8,912				8,912
Stock-based compensation expense	6,648		6,648
Issuance of common stock related to options exercised, net of shares withheld for payroll taxes (in shares)		26,784
Issuance of common stock related to options exercised, net of shares withheld for payroll taxes	332		332
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes (in shares)		(13,995)
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes		$ 1	(1)
Vesting of performance based stock awards, net of shares withheld for payroll taxes (in shares)		17,655
Time based stock awards granted (in shares)		112,636
Issuance of common stock related to employee stock purchase plan (in shares)		11,723
Issuance of common stock related to employee stock purchase plan	585		585
Cash dividends declared	(14,926)			(14,926)
Common stock repurchased (in shares)		(822,200)
Common stock repurchased	(33,385)	$ (7)	(33,378)
Ending balance (in shares) at Sep. 30, 2019		33,720,522
Ending balance at Sep. 30, 2019	1,184,594	$ 336	502,566	677,055	4,637
Beginning balance (in shares) at Jun. 30, 2019		34,539,853
Beginning balance at Jun. 30, 2019	1,184,582	$ 343	532,585	647,887	3,767
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	36,495			36,495
Other comprehensive income, net of tax	870				870
Stock-based compensation expense	3,147		3,147
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes (in shares)		(1,251)
Issuance of common stock related to employee stock purchase plan (in shares)		4,120
Issuance of common stock related to employee stock purchase plan	213		213
Cash dividends declared	(7,327)			(7,327)
Common stock repurchased (in shares)		(822,200)
Common stock repurchased	(33,386)	$ (7)	(33,379)
Ending balance (in shares) at Sep. 30, 2019		33,720,522
Ending balance at Sep. 30, 2019	1,184,594	$ 336	502,566	677,055	4,637
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment due to the adoption of ASC 326, net of tax	$ 10,614
Beginning balance (in shares) at Dec. 31, 2019	33,241,496	33,241,496
Beginning balance at Dec. 31, 2019	$ 1,190,681	$ 331	482,286	705,105	2,959
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	93,325	$ 0	0	93,325	0
Other comprehensive income, net of tax	11,312				11,312
Stock-based compensation expense	3,874		3,874
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes (in shares)		(28,218)
Vesting of performance based stock awards, net of shares withheld for payroll taxes (in shares)		4,126
Time based stock awards granted (in shares)		176,252
Issuance of common stock related to employee stock purchase plan (in shares)		17,821
Issuance of common stock related to employee stock purchase plan	589		589
Cash dividends declared	(21,280)			(21,280)
Common stock repurchased (in shares)		(1,182,841)
Common stock repurchased	$ (44,168)	$ (11)	(44,157)
Ending balance (in shares) at Sep. 30, 2020	32,228,636	32,228,636
Ending balance at Sep. 30, 2020	$ 1,223,402	$ 320	442,592	766,219	14,271
Beginning balance (in shares) at Jun. 30, 2020		32,224,756
Beginning balance at Jun. 30, 2020	1,187,895	$ 320	440,934	731,973	14,668
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	41,346			41,346
Other comprehensive income, net of tax	(397)				(397)
Stock-based compensation expense	1,452		1,452
Vesting of time based stock awards issued at date of grant, net of shares withheld for payroll taxes (in shares)		(3,297)
Issuance of common stock related to employee stock purchase plan (in shares)		7,177
Issuance of common stock related to employee stock purchase plan	206		206
Cash dividends declared	$ (7,100)			(7,100)
Ending balance (in shares) at Sep. 30, 2020	32,228,636	32,228,636
Ending balance at Sep. 30, 2020	$ 1,223,402	$ 320	$ 442,592	766,219	$ 14,271
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect adjustment due to the adoption of ASC 326, net of tax	$ (10,931)			$ (10,931)