Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding (in shares)	19,253	20,222	19,859	20,475
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock (in shares)	8	3	9	5
Weighted average dilutive common shares outstanding (in shares)	19,261	20,225	19,868	20,480
Basic:
Net income	$ 34,778	$ 29,881	$ 127,054	$ 102,953
Less: Earnings allocated to participating securities	1,182	1,024	4,308	3,660
Net income available to basic common shares	$ 33,596	$ 28,857	$ 122,746	$ 99,293
Basic earnings per common share (in dollars per share)	$ 1.74	$ 1.43	$ 6.18	$ 4.85
Diluted:
Net income	$ 34,778	$ 29,881	$ 127,054	$ 102,953
Less: Earnings allocated to participating securities	1,181	1,023	4,306	3,659
Net income available to diluted common shares	$ 33,597	$ 28,858	$ 122,748	$ 99,294
Diluted earnings per common share (in dollars per share)	$ 1.74	$ 1.43	$ 6.18	$ 4.85