Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2015
Accounting Policies [Abstract]
Net income attributable to common stockholders (Dilutive)	$ 22,524	$ 18,615
Stock options, SARs and restricted stock	119	231
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	10,782
Diluted earnings per common share	$ 2.1000	$ 1.73000
Earnings Per Share, Basic [Abstract]
Net Income (Loss) Available to Common Stockholders, Basic	$ 22,524	$ 18,615	$ 47,371
Weighted Average Number of Shares Outstanding, Basic	10,622	10,551
Basic earnings per common share	$ 2.12	$ 1.76