Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 26,340	$ 17,488	$ 48,864	$ 36,103
Net Income (Loss) Available to Common Stockholders, Diluted	$ 26,340	$ 17,488	$ 48,864	$ 36,103
Stock options, SARs and restricted stock	84,000	183,000	102,000	207,000
Weighted Average Number of Shares Outstanding, Basic	10,658,000	10,600,000	10,640,000	10,576,000
Basic earnings per common share	$ 2.47	$ 1.65	$ 4.59	$ 3.41
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements	$ (0.02)	$ (0.03)	$ (0.04)	$ (0.06)
Weighted Average Number of Shares Outstanding, Diluted	10,742,000	10,783,000	10,742,000	10,783,000
Diluted earnings per common share	$ 2.45000	$ 1.62000	$ 4.55000	$ 3.35
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)	0	24,907	25,062	37,832
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)	13,138	32,892	13,138	32,892