Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Computation of basic and diluted net income per common share
Net income	$ 26,067	$ 23,515	$ 61,357	$ 86,506
Net income per common share - basic (in dollars per share)	$ 0.55	$ 0.50	$ 1.29	$ 1.85
Net income per common share - diluted (in dollars per share)	$ 0.55	$ 0.49	$ 1.29	$ 1.82
Weighted average common shares outstanding - basic (in shares)	47,476	46,677	47,406	46,701
Plus dilutive effect of potential common shares (in shares)	200	930	241	899
Weighted average common shares outstanding - diluted (in shares)	47,676	47,607	47,647	47,600
Common stock excluded from calculation of EPS (in shares)	84	32	79	55