Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment for adoption of ASU 2016-01				$ 0
Adjusted equity balance				593
Balance at beginning of period at Dec. 31, 2016	$ 550,044	$ 510,861	$ 38,590	593
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	39,405		39,405
Other comprehensive income, net of tax	188			188
Stock based compensation	1,982	1,982
Exercise of stock options	313	313
Retirement of restricted stock	(773)	(773)
Cash dividends, common stock	(14,078)		(14,078)
Balance at end of period at Sep. 30, 2017	577,081	512,383	63,917	781
Balance at beginning of period at Jun. 30, 2017	567,545	511,967	54,969	609
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	13,723		13,723
Other comprehensive income, net of tax	172			172
Stock based compensation	416	416
Exercise of stock options	0
Retirement of restricted stock	0
Cash dividends, common stock	(4,775)		(4,775)
Balance at end of period at Sep. 30, 2017	577,081	512,383	63,917	781
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment for adoption of ASU 2016-01	0		2,043	(2,043)
Adjusted equity balance	583,122	512,734	74,780	(4,392)
Balance at beginning of period at Dec. 31, 2017	583,122	512,734	72,737	(2,349)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,851
Other comprehensive income, net of tax	(9,099)			(9,099)
Stock based compensation	1,930	1,930
Exercise of stock options	307	307
Retirement of restricted stock	(759)	(759)
Cash dividends, common stock	(15,797)		(15,797)
Balance at end of period at Sep. 30, 2018	607,555	514,212	106,834	(13,491)
Balance at beginning of period at Jun. 30, 2018	597,864	513,756	95,586	(11,478)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,758		16,758
Other comprehensive income, net of tax	(2,013)			(2,013)
Stock based compensation	471	471
Exercise of stock options	0
Retirement of restricted stock	(15)	(15)
Cash dividends, common stock	(5,510)		(5,510)
Balance at end of period at Sep. 30, 2018	$ 607,555	$ 514,212	$ 106,834	$ (13,491)