Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic EPS
Net Income	$ 14,406	$ 26,474	$ 64,584	$ 75,338
Weighted-Average Shares (in shares)	10,706,000	10,668,000	10,691,000	10,650,000
Dollars per share (in dollars per share)	$ 1.34	$ 2.48	$ 6.04	$ 7.07
Effect of dilutive securities
SARs and restricted stock (in shares)	70,000	76,000	83,000	93,000
SARs and restricted stock (in dollars per share)	$ (0.01)	$ (0.02)	$ (0.05)	$ (0.06)
Net income, Diluted EPS	$ 14,406	$ 26,474	$ 64,584	$ 75,338
Weighted-Average Shares, Diluted EPS (in shares)	10,776,000	10,744,000	10,774,000	10,743,000
Diluted EPS (in dollars per share)	$ 1.33	$ 2.46	$ 5.99	$ 7.01
Stock Appreciation Rights [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	26,768		48,801
Restricted Stock [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)		10,122		15,437
Performance Shares [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	8,414	13,138	10,994	13,138