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	25,682		25,682
Other comprehensive income, net of tax	16			16
Stock based compensation	1,566	1,566
Exercise of stock options	313	313
Retirement of restricted stock	(773)	(773)
Cash dividends, common stock	(9,303)		(9,303)
Balance at end of period at Jun. 30, 2017	567,545	511,967	54,969	609
Balance at beginning of period at Mar. 31, 2017				(308)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	13,370
Other comprehensive income, net of tax	917
Balance at end of period at Jun. 30, 2017	567,545	511,967	54,969	609
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment for adoption of ASU 2016-01	0	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	31,093		31,093
Other comprehensive income, net of tax	(7,086)			(7,086)
Stock based compensation	1,459	1,459
Exercise of stock options	307	307
Retirement of restricted stock	(744)	(744)
Cash dividends, common stock	(10,287)		(10,287)
Balance at end of period at Jun. 30, 2018	597,864	513,756	95,586	(11,478)
Balance at beginning of period at Mar. 31, 2018				(9,841)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	15,838
Other comprehensive income, net of tax	(1,637)
Balance at end of period at Jun. 30, 2018	$ 597,864	$ 513,756	$ 95,586	$ (11,478)