Accounting Policies - Earnings Per Share Anti-Dilutive (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 25,465	$ 16,364	$ 12,006	$ 10,317	$ 15,032	$ 8,359	$ 22,162	$ 1,818	$ 64,152	$ 47,371	$ 38,251
Net Income (Loss) Available to Common Stockholders, Diluted									$ 64,152	$ 47,371	$ 38,251
Stock options, SARs and restricted stock									269,000	360,000	447,000
Weighted Average Number of Shares Outstanding, Basic									10,477,000	10,949,000	10,920,000
Basic earnings per common share	$ 2.42	$ 1.56	$ 1.15	$ 0.99	$ 1.39	$ 0.76	$ 2.01	$ 0.17	$ 6.12	$ 4.33	$ 3.50
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements									$ (0.15)	$ (0.14)	$ (0.13)
Weighted Average Number of Shares Outstanding, Diluted									10,746,000	11,309,000	11,367,000
Diluted earnings per common share	$ 2.38	$ 1.54	$ 1.13	$ 0.94	$ 1.35	$ 0.74	$ 1.94	$ 0.16	$ 5.97000	$ 4.19000	$ 3.37000
Stock Options and SARs [Members]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)									86,907	304,132	109,143
Performance Shares [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (shares)									37,284	46,303	36,784