Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Interest and dividend income	$ 59,029	$ 57,772	$ 56,342	$ 53,621	$ 51,936	$ 51,077	$ 50,417	$ 49,309	$ 226,764	$ 202,739	$ 188,746
Interest expense	7,060	6,772	6,041	5,587	5,562	5,760	5,510	5,271	25,460	22,103	20,365
Net interest income	51,969	51,000	50,301	48,034	46,374	45,317	44,907	44,038	201,304	180,636	168,381
Provision for loan and lease losses	2,014	402	976	769	1,706	782	978	855	4,161	4,321	2,945
Income before income tax expense	20,981	25,917	25,207	23,137	21,595	22,116	21,625	21,180	95,242	86,516	87,514
Net Income	$ 2,457	$ 17,394	$ 16,926	$ 15,717	$ 15,118	$ 15,138	$ 14,833	$ 14,251	$ 52,494	$ 59,340	$ 58,421
Net income per common share (basic) (in dollars per share)	$ 0.16	$ 1.14	$ 1.11	$ 1.04	$ 1.00	$ 1.01	$ 0.99	$ 0.95	$ 3.46	$ 3.94	$ 3.91
Net income per common share (diluted) (in dollars per share)	$ 0.16	$ 1.14	$ 1.11	$ 1.03	$ 0.99	$ 1.00	$ 0.98	$ 0.94	$ 3.43	$ 3.91	$ 3.87