Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Accounting Policies [Abstract]
Net income attributable to common stockholders (Dilutive)									$ 64,152	$ 47,371	$ 38,251
Stock options, SARs and restricted stock									269	360	447
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	11,309	11,367
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
Earnings Per Share, Basic [Abstract]
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
Weighted Average Number of Shares Outstanding, Basic									10,477	10,949	10,920
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