Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding	20,298	20,692
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock	9	6
Weighted average dilutive common shares outstanding	20,307	20,698
Basic:
Net income	$ 35,814	$ 33,939
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic	1,209	1,250
Earnings available to basic common shares	$ 34,605	$ 32,689
Basic earnings per common share	$ 1.70	$ 1.58
Diluted:
Net income	$ 35,814	$ 33,939
Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted	1,208	1,250
Earnings available to diluted common shares	$ 34,606	$ 32,689
Diluted earnings per common share	$ 1.70	$ 1.58