Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Undistributed Earnings (Loss) Allocated to Participating Securities, Diluted									$ 7,511	$ 5,869	$ 3,595
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding									20,420	21,161	23,148
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock									5	9	4
Weighted average dilutive common shares outstanding									20,425	21,170	23,152
Basic:
Net income									$ 213,442	$ 147,065	$ 93,999
Undistributed Earnings (Loss) Allocated to Participating Securities, Basic									7,512	5,871	3,595
Earnings available to basic common shares									$ 205,930	$ 141,194	$ 90,404
Basic earnings per common share	$ 5.27	$ 1.43	$ 1.84	$ 1.58	$ 1.44	$ 1.65	$ 2.12	$ 1.47	$ 10.08	$ 6.67	$ 3.91
Diluted:
Net income									$ 213,442	$ 147,065	$ 93,999
Earnings available to diluted common shares									$ 205,931	$ 141,196	$ 90,404
Diluted earnings per common share	$ 5.27	$ 1.43	$ 1.84	$ 1.58	$ 1.44	$ 1.65	$ 2.12	$ 1.47	$ 10.08	$ 6.67	$ 3.90