Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 19,560	$ 26,474	$ 26,340	$ 22,524	$ 16,710	$ 18,487	$ 17,488	$ 18,615	$ 94,898	$ 71,300	$ 64,152
Net Income (Loss) Available to Common Stockholders, Diluted									$ 94,898	$ 71,300	$ 64,152
Stock options, SARs and restricted stock									92,000	209,000	269,000
Weighted Average Number of Shares Outstanding, Basic									10,654,000	10,594,000	10,477,000
Basic earnings per common share	$ 1.84	$ 2.48	$ 2.47	$ 2.12	$ 1.57	$ 1.74	$ 1.65	$ 1.76	$ 8.91	$ 6.73	$ 6.12
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements									$ (0.08)	$ (0.13)	$ (0.15)
Weighted Average Number of Shares Outstanding, Diluted									10,746,000	10,803,000	10,746,000
Diluted earnings per common share	$ 1.82	$ 2.46	$ 2.45	$ 2.10	$ 1.55	$ 1.71	$ 1.62	$ 1.73	$ 8.83000	$ 6.6000	$ 5.97000
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)									15,812	28,579	86,907
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)									13,138	29,647	37,284