Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic and Diluted Earnings per share
Weighted average basic common shares outstanding (in shares)	20,036	20,516	20,167	20,604
Dilutive effect of employee stock purchases, net of assumed repurchase of treasury stock (in shares)	10	6	9	5
Weighted average dilutive common shares outstanding (in shares)	20,046	20,522	20,176	20,609
Basic:
Net income	$ 56,463	$ 39,133	$ 92,277	$ 73,072
Less: Earnings allocated to participating securities	1,916	1,389	3,123	2,645
Net income available to basic common shares	$ 54,547	$ 37,744	$ 89,154	$ 70,427
Basic earnings per common share (in dollars per share)	$ 2.72	$ 1.84	$ 4.42	$ 3.42
Diluted:
Net income	$ 56,463	$ 39,133	$ 92,277	$ 73,072
Less: Earnings allocated to participating securities	1,916	1,389	3,123	2,645
Net income available to diluted common shares	$ 54,547	$ 37,744	$ 89,154	$ 70,427
Diluted earnings per common share (in dollars per share)	$ 2.72	$ 1.84	$ 4.42	$ 3.42