Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Accounting Policies [Abstract]
Net income attributable to common stockholders (Dilutive)									$ 71,300	$ 64,152	$ 47,371
Stock options, SARs and restricted stock									209	269	360
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	10,746	11,309
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
Earnings Per Share, Basic [Abstract]
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
Weighted Average Number of Shares Outstanding, Basic									10,594	10,477	10,949
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