Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Basic EPS
Net Income	$ 21,874	$ 22,524
Weighted-Average Shares (in shares)	10,670,000	10,622,000
Dollars per share (in dollars per share)	$ 2.05	$ 2.12
Effect of dilutive securities
SARs and restricted stock (in shares)	107,000	119,000
SARs and restricted stock (in dollars per share)	$ (0.02)	$ (0.02)
Net income, Diluted EPS	$ 21,874	$ 22,524
Weighted-Average Shares, Diluted EPS (in shares)	10,777,000	10,741,000
Diluted EPS (in dollars per share)	$ 2.03	$ 2.10
Stock Appreciation Rights [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	56,976	56,976
Restricted Stock [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)		25,062
Performance Shares [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	12,284	13,138