Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balance at Dec. 31, 2015	$ 452,116	$ 247,587	$ 206,307	$ (1,778)
Beginning balance, shares at Dec. 31, 2015		22,775,173
Net income	44,811		44,811
Other comprehensive income (loss)	(6,135)			(6,135)
Stock option vesting	580	$ 580
Service condition RSU vesting	616	616
Market plus service condition RSU vesting	271	271
Stock options exercised	6,506	$ 6,506
Stock options exercised, shares		336,900
Tax effect of stock options exercised	154	$ 154
Service condition RSUs released	0	$ 0	0	0
Service condition RSUs released, shares		20,529
Tax benefit from release of service condition RSUs	1	$ 1
Repurchase of common stock, value	(7,878)	$ (2,895)	(4,983)
Repurchase of common stock, shares		(264,800)
Dividends paid	(13,695)		(13,695)
Ending balance at Dec. 31, 2016	477,347	$ 252,820	232,440	(7,913)
Ending balance, shares at Dec. 31, 2016		22,867,802
Net income	40,554		40,554
Other comprehensive income (loss)	2,685			2,685
Stock option vesting	259	$ 259
Service condition RSU vesting	895	895
Market plus service condition RSU vesting	432	432
Stock options exercised	2,621	$ 2,621
Stock options exercised, shares		145,850
Service condition RSUs released	0	$ 0	0	0
Service condition RSUs released, shares		30,896
Market plus service condition RSUs released	0	$ 0	0	0
Market plus service conditions RSUs released, shares		18,805
Repurchase of common stock, value	(3,854)	$ (1,191)	(2,663)
Repurchase of common stock, shares		(107,390)
Dividends paid	(15,131)		(15,131)
Ending balance at Dec. 31, 2017	$ 505,808	$ 255,836	255,200	(5,228)
Ending balance, shares at Dec. 31, 2017	22,955,963	22,955,963
Net income	$ 68,320		68,320
Adoption ASU | ASU 2016-01 [Member]			(62)	62
Adoption ASU | ASU 2018-02 [Member]			1,093	(1,093)
Other comprehensive income (loss)	(11,620)			(11,620)
Stock option vesting	75	$ 75
Service condition RSU vesting	1,017	1,017
Market plus service condition RSU vesting	370	370
Stock options exercised	$ 1,704	$ 1,704
Stock options exercised, shares	100,400	100,400
Service condition RSUs released	$ 0	$ 0	0	0
Service condition RSUs released, shares		35,060
Market plus service condition RSUs released	0	$ 0	0	0
Market plus service conditions RSUs released, shares		25,512
Issuance of common stock	284,437	$ 284,437
Issuance of common stock, shares		7,405,277
Repurchase of common stock, value	(3,969)	$ (1,677)	(2,292)
Repurchase of common stock, shares		(104,989)
Dividends paid	(18,769)		(18,769)
Ending balance at Dec. 31, 2018	$ 827,373	$ 541,762	$ 303,490	$ (17,879)
Ending balance, shares at Dec. 31, 2018	30,417,223	30,417,223