Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Common Stock [Member]	Retained Earnings [Member]	Stock in Rabbi Trust Assumed in Acquisition [Member]	Rabbi Trust Obligation [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Total
Beginning balance at Dec. 31, 2016	$ 147,287	$ 225,921			$ (5,107)	$ 368,101
Beginning balance, shares at Dec. 31, 2016	20,845
Net income		31,795				31,795
Cash dividends declared		(5,926)				(5,926)
Equity issued pursuant to acquisition	$ 114,893		(7,688)	7,688		114,893
Equity issued pursuant to acquisition, shares	3,813
Payment of deferred fees			4,117	(4,117)
Stock option exercises	$ 287					287
Stock option exercises, shares	16
Stock-based compensation	$ 1,026					1,026
Stock-based compensation, shares	50
Other comprehensive income (loss)					2,325	2,325
Ending balance at Sep. 30, 2017	$ 263,493	251,790	(3,571)	3,571	(2,782)	$ 512,501
Ending balance, shares at Sep. 30, 2017	24,724					24,723,929
Beginning balance at Dec. 31, 2017	$ 432,794	264,331	(3,581)	3,581	(4,146)	$ 692,979
Beginning balance, shares at Dec. 31, 2017	29,639					29,639,374
Net income		65,394				$ 65,394
Cash dividends declared		(8,903)				(8,903)
Payment of deferred fees			357	(357)
Stock option exercises	$ 324					324
Stock option exercises, shares	25
Stock withheld for payment of taxes	$ (264)					(264)
Stock withheld for payment of taxes, shares	(7)
Stock-based compensation	$ 1,373					1,373
Stock-based compensation, shares	72
Other comprehensive income (loss)					(7,827)	(7,827)
Ending balance at Sep. 30, 2018	$ 434,227	$ 320,822	$ (3,224)	$ 3,224	$ (11,973)	$ 743,076
Ending balance, shares at Sep. 30, 2018	29,729					29,729,285