Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 22,524	$ 18,615	$ 47,371
Net Income (Loss) Available to Common Stockholders, Diluted	$ 22,524	$ 18,615
Stock options, SARs and restricted stock	119,000	231,000
Weighted Average Number of Shares Outstanding, Basic	10,622,000	10,551,000
Basic earnings per common share	$ 2.12	$ 1.76
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements	$ (0.02)	$ (0.03)
Weighted Average Number of Shares Outstanding, Diluted	10,741,000	10,782,000
Diluted earnings per common share	$ 2.1000	$ 1.73000
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)	25,062	50,757
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