Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 18,487		$ 16,364		$ 54,590		$ 38,687
Net Income (Loss) Available to Common Stockholders, Diluted	$ 18,487		$ 16,364		$ 54,590		$ 38,687
Stock options, SARs and restricted stock	210,000	[1]	176,000	[2]	208,000	[2]	291,000	[2]
Weighted Average Number of Shares Outstanding, Basic	10,605,000		10,473,000		10,586,000		10,464,000
Basic earnings per common share	$ 1.74	[3]	$ 1.56		$ 5.16	[4]	$ 3.70
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements	$ (0.03)	[1]	$ (0.02)	[2]	$ (0.10)	[2]	$ (0.10)	[2]
Weighted Average Number of Shares Outstanding, Diluted	10,815,000		10,649,000		10,794,000		10,755,000
Diluted earnings per common share	$ 1.71000	[3]	$ 1.54000		$ 5.06000	[4]	$ 3.6000
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)	24,657		54,709		33,440		115,875
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)	32,892		37,284		32,892		37,284

[1] For the three months ended September 30, 2017 and 2016, stock options and SARs of 24,657 and 54,709, respectively, were outstanding but not included in the computation of diluted earnings per share of common stock because they were anti-dilutive. For the three months ended September 30, 2017 and 2016, contingent shares of non-vested restricted stock of 32,892, and 37,284, respectively, were outstanding but not included in the computation of diluted earnings per share of common stock because performance conditions had not yet been met.

[2] For the nine months ended September 30, 2017 and 2016, stock options and SARs of 33,440 and 115,875, respectively, were outstanding but not included in the computation of diluted earnings per share of common stock because they were anti-dilutive. For the nine months ended September 30, 2017 and 2016, contingent shares of non-vested restricted stock of 32,892, and 37,284, respectively, were outstanding but not included in the computation of diluted earnings per share of common stock because performance conditions had not yet been met.

[3] For the effect of the adoption of the new Accounting Standard Update 2016-09, "Improvements to Employee Share-Based Payment Accounting," on Basic and Diluted EPS, see Note 1(d) "New Accounting Standards."

[4] For the effect of the adoption of the new Accounting Standard Update 2016-09, "Improvements to Employee Share-Based Payment Accounting," on Basic and Diluted EPS, see Note 1(d) "New Accounting Standards."