Condensed 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, 2016	$ 477,347	$ 252,820	$ 232,440	$ (7,913)
Beginning balance, shares at Dec. 31, 2016		22,867,802
Net income	37,565		37,565
Other comprehensive income (loss)	3,301			3,301
Stock option vesting	211	$ 211
Service condition RSU vesting	657	657
Market plus service condition RSU vesting	316	316
Service condition RSUs released	0	$ 0	0	0
Service condition RSUs released, shares		28,397
Market plus service condition RSUs released	0	$ 0	0	0
Market plus service conditions RSUs released, shares		18,805
Stock options exercised	2,418	$ 2,418
Stock options exercised, shares		133,850
Repurchase of common stock, value	(3,854)	$ (1,191)	(2,663)
Repurchase of common stock, shares		(107,390)
Dividends paid	(11,228)		(11,228)
Ending balance at Sep. 30, 2017	506,733	$ 255,231	256,114	(4,612)
Ending balance, shares at Sep. 30, 2017		22,941,464
Beginning balance at Dec. 31, 2017	$ 505,808	$ 255,836	255,200	(5,228)
Beginning balance, shares at Dec. 31, 2017	22,955,963	22,955,963
Net income	$ 45,109		45,109
Adoption ASU | ASU 2016-01 [Member]			(62)	62
Adoption ASU | ASU 2018-02 [Member]			1,093	(1,093)
Other comprehensive income (loss)	(20,700)			(20,700)
Stock option vesting	75	$ 75
Service condition RSU vesting	745	745
Market plus service condition RSU vesting	274	274
Service condition RSUs released	0	$ 0	0	0
Service condition RSUs released, shares		32,516
Market plus service condition RSUs released	0	$ 0	0	0
Market plus service conditions RSUs released, shares		25,512
Stock options exercised	$ 475	$ 475
Stock options exercised, shares	27,400	27,400
Issuance of common stock	$ 284,437	$ 284,437
Issuance of common stock, shares		7,405,277
Repurchase of common stock, value	(1,124)	$ (323)	(801)
Repurchase of common stock, shares		(28,850)
Dividends paid	(12,984)		(12,984)
Ending balance at Sep. 30, 2018	$ 802,115	$ 541,519	$ 287,555	$ (26,959)
Ending balance, shares at Sep. 30, 2018	30,417,818	30,417,818