Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Capital Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss), Net of Tax	Accumulated Other Comprehensive Income (Loss), Net of Tax Interest Rate Swap	Accumulated Other Comprehensive Income (Loss), Net of Tax Available-for-sale Securities	Treasury Stock
Balance at beginning of period (in shares) at Dec. 31, 2014		28,159,027						1,385,164
Balance at beginning of period at Dec. 31, 2014		$ 28,159	$ 225,015	$ 118,412		$ 508	$ 5,590	$ (11,656)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)		5,320,000
Issuance of common stock		$ 5,320	109,569
Exercise of stock options (in shares)		75,135
Exercise of stock options		$ 75	1,116
Issuance of restricted shares (in shares)		71,000
Issuance of restricted shares		$ 71	(71)
Forfeitures of restricted shares (in shares)		0
Forfeitures of restricted shares		$ 0	0
Share-based compensation			1,485
Share-based compensation net tax benefit			235
Net income	$ 40,847			40,847
Dividends on common shares				(6,439)
Change during period	(2,745)					(356)	(2,389)
Purchase of treasury shares (in shares)								28,613
Purchase of treasury shares								$ (732)
Balance at end of period (in shares) at Dec. 31, 2015		33,625,162						1,413,777
Balance at end of period at Dec. 31, 2015	514,759	$ 33,625	337,349	152,820	$ 3,353	152	3,201	$ (12,388)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)		2,549,469
Issuance of common stock		$ 2,549	69,906
Exercise of stock options (in shares)		54,510
Exercise of stock options		$ 55	909
Issuance of restricted shares (in shares)		155,751
Issuance of restricted shares		$ 156	(156)
Forfeitures of restricted shares (in shares)		(7,085)
Forfeitures of restricted shares		$ (7)	7
Share-based compensation			2,261
Share-based compensation net tax benefit			0
Net income	72,100			72,100
Dividends on common shares				(10,466)
Change during period	(4,411)					24	(4,435)
Purchase of treasury shares (in shares)								42,556
Purchase of treasury shares								$ (1,225)
Balance at end of period (in shares) at Dec. 31, 2016		36,377,807						1,456,333
Balance at end of period at Dec. 31, 2016	646,437	$ 36,378	410,276	214,454	(1,058)	176	(1,234)	$ (13,613)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock (in shares)		2,141,072
Issuance of common stock		$ 2,141	92,359
Exercise of stock options (in shares)		132,319
Exercise of stock options		$ 132	2,537
Issuance of restricted shares (in shares)		84,147
Issuance of restricted shares		$ 84	(84)
Forfeitures of restricted shares (in shares)		(472)
Forfeitures of restricted shares		$ 0	0
Share-based compensation			3,316
Share-based compensation net tax benefit			0
Net income	73,548			73,548
Dividends on common shares				(14,883)
Change during period	(222)					116	(338)
Purchase of treasury shares (in shares)								18,528
Purchase of treasury shares								$ (886)
Balance at end of period (in shares) at Dec. 31, 2017		38,734,873						1,474,861
Balance at end of period at Dec. 31, 2017	$ 804,479	$ 38,735	$ 508,404	$ 273,119	$ (1,280)	$ 292	$ (1,572)	$ (14,499)