Earnings Per Common Share - Computation of basic and diluted earnings per common share (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended						9 Months Ended
		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic Earnings Per Share:
Net income available to common shareholders		$ 13,171	$ 12,199	$ 11,482	$ 11,306	$ 11,109	$ 11,737	$ 36,852	$ 34,152
Less: Income allocated to participating securities		1			1			3	3
Net income allocated to common shareholders		$ 13,170			$ 11,305			$ 36,849	$ 34,149
Weighted average common shares outstanding including participating securities	[1]	19,801			20,409			19,873	20,382
Less: Participating securities		11			9			11	9
Average common shares	[1]	19,790			20,400			19,862	20,373
Basic earnings per common share		$ 0.67			$ 0.55			$ 1.86	$ 1.68
Diluted Earnings Per Share:
Net income allocated to common shareholders		$ 13,170			$ 11,305			$ 36,849	$ 34,149
Weighted average common shares outstanding for basic earnings per common share	[1]	19,790			20,400			19,862	20,373
Add: Dilutive effects of stock options		85			67			81	92
Average shares and dilutive potential common shares	[1]	19,875			20,467			19,943	20,465
Diluted earnings per common share		$ 0.66			$ 0.55			$ 1.85	$ 1.67

[1] Share and per share data in above table has been adjusted for a 2-for-1 stock split on July 12, 2018.