Earnings Per Share - Calculation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Earnings Per Share [Abstract]
Net income	$ 17,418	$ 20,667	$ 15,253	$ 23,543	$ 18,771	$ 14,583	$ 53,338	$ 56,897
Average Common Shares:
Basic (weighted-average outstanding shares) (in shares)	11,748			11,689			11,724	11,649
Dilutive potential common shares from stock options (in shares)	65			88			72	109
Diluted (weighted-average outstanding shares) (in shares)	11,813			11,777			11,796	11,758
Basic earnings per share (in dollars per share)	$ 1.48			$ 2.01			$ 4.55	$ 4.88
Diluted earnings per share (in dollars per share)	$ 1.47			$ 2.00			$ 4.52	$ 4.84