Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding	21,057	23,312	21,753	23,377
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock	13	3	9	3
Weighted average dilutive common shares outstanding	21,070	23,315	21,762	23,380
Basic:
Net income	$ 46,580	$ 46,310	$ 80,871	$ 82,124
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic	1,885	1,792	3,224	3,176
Earnings available to basic common shares	$ 44,695	$ 44,518	$ 77,647	$ 78,948
Basic earnings per common share	$ 2.12	$ 1.91	$ 3.57	$ 3.38
Diluted:
Net income	$ 46,580	$ 46,310	$ 80,871	$ 82,124
Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted	1,884	1,792	3,223	3,176
Earnings available to diluted common shares	$ 44,696	$ 44,518	$ 77,648	$ 78,948
Diluted earnings per common share	$ 2.12	$ 1.91	$ 3.57	$ 3.38