Accounting Policies - Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Net income attributable to common stockholders (Dilutive)									$ 94,898	$ 71,300	$ 64,152
Stock options, SARs and restricted stock									92	209	269
Earnings Per Share, On Incremental Common Shares Attributable to Share-based Payment Arrangements									$ (0.08)	$ (0.13)	$ (0.15)
Weighted Average Number of Shares Outstanding, Diluted									10,746	10,803	10,746
Diluted earnings per common share	$ 1.82	$ 2.46	$ 2.45	$ 2.10	$ 1.55	$ 1.71	$ 1.62	$ 1.73	$ 8.83000	$ 6.6000	$ 5.97000
Earnings Per Share, Basic [Abstract]
Net Income (Loss) Available to Common Stockholders, Basic	$ 19,560	$ 26,474	$ 26,340	$ 22,524	$ 16,710	$ 18,487	$ 17,488	$ 18,615	$ 94,898	$ 71,300	$ 64,152
Weighted Average Number of Shares Outstanding, Basic									10,654	10,594	10,477
Basic earnings per common share	$ 1.84	$ 2.48	$ 2.47	$ 2.12	$ 1.57	$ 1.74	$ 1.65	$ 1.76	$ 8.91	$ 6.73	$ 6.12