Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 16,710	$ 18,487	$ 17,488	$ 18,615	$ 25,465	$ 16,364	$ 12,006	$ 10,317	$ 71,300	$ 64,152	$ 47,371
Net Income (Loss) Available to Common Stockholders, Diluted									$ 71,300	$ 64,152	$ 47,371
Stock options, SARs and restricted stock									209,000	269,000	360,000
Weighted Average Number of Shares Outstanding, Basic									10,594,000	10,477,000	10,949,000
Basic earnings per common share	$ 1.57	$ 1.74	$ 1.65	$ 1.76	$ 2.42	$ 1.56	$ 1.15	$ 0.99	$ 6.73	$ 6.12	$ 4.33
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements									$ (0.13)	$ (0.15)	$ (0.14)
Weighted Average Number of Shares Outstanding, Diluted									10,803,000	10,746,000	11,309,000
Diluted earnings per common share	$ 1.55	$ 1.71	$ 1.62	$ 1.73	$ 2.38	$ 1.54	$ 1.13	$ 0.94	$ 6.6000	$ 5.97000	$ 4.19000
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)									28,579	86,907	304,132
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)									29,647	37,284	46,303