Consolidated statements of changes in shareholders' equity - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income, net
Balance at Dec. 31, 2017	$ 596,729	$ 30,536	$ 418,596	$ 147,449	$ 148
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	63,196			63,196
Other comprehensive (loss) income, net of taxes	(11,262)				(11,262)
Stock based compensation expense	5,327	9	5,318
Restricted stock units vested and distributed, net of shares withheld	(2,637)	142	(2,779)
Shares issued under employee stock purchase program	1,196	29	1,167
Cash dividends declared	(3,818)			(3,818)
Balance at Sep. 30, 2018	648,731	30,716	422,302	206,718	(11,005)
Balance at Jun. 30, 2018	630,959	30,683	420,382	187,250	(7,356)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,377			21,377
Other comprehensive (loss) income, net of taxes	(3,649)				(3,649)
Stock based compensation expense	1,508	3	1,505
Restricted stock units vested and distributed, net of shares withheld	(82)	18	(100)
Shares issued under employee stock purchase program	527	12	515
Cash dividends declared	(1,909)			(1,909)
Balance at Sep. 30, 2018	648,731	30,716	422,302	206,718	(11,005)
Balance at Dec. 31, 2018	671,857	30,725	424,146	221,213	(4,227)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	62,242			62,242
Other comprehensive (loss) income, net of taxes	16,827				16,827
Stock based compensation expense	5,621	9	5,612
Restricted stock units vested and distributed, net of shares withheld	(3,552)	171	(3,723)
Shares issued under employee stock purchase program	804	23	781
Cash dividends declared	(7,655)			(7,655)
Balance at Sep. 30, 2019	744,835	30,928	426,816	274,491	12,600
Balance at Jun. 30, 2019	718,759	30,866	425,644	253,080	9,169
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	23,966			23,966
Other comprehensive (loss) income, net of taxes	3,431				3,431
Stock based compensation expense	1,836	3	1,833
Restricted stock units vested and distributed, net of shares withheld	(1,042)	47	(1,089)
Shares issued under employee stock purchase program	440	12	428
Cash dividends declared	(2,555)			(2,555)
Balance at Sep. 30, 2019	$ 744,835	$ 30,928	$ 426,816	$ 274,491	$ 12,600