Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 26,474	$ 18,487	$ 75,338	$ 54,590
Net Income (Loss) Available to Common Stockholders, Diluted	$ 26,474	$ 18,487	$ 75,338	$ 54,590
Stock options, SARs and restricted stock	76,000	210,000	93,000	208,000
Weighted Average Number of Shares Outstanding, Basic	10,668,000	10,605,000	10,650,000	10,586,000
Basic earnings per common share	$ 2.48	$ 1.74	$ 7.07	$ 5.16
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements	$ (0.02)	$ (0.03)	$ (0.06)	$ (0.10)
Weighted Average Number of Shares Outstanding, Diluted	10,744,000	10,815,000	10,743,000	10,794,000
Diluted earnings per common share	$ 2.46000	$ 1.71000	$ 7.01000	$ 5.06
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)	10,122	24,657	15,437	33,440
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