Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic EPS
Net income attributable to common stockholders	$ 18,659	$ 14,406	$ 59,745	$ 64,584
Weighted-Average Shares (in shares)	10,734,000	10,706,000	10,725,000	10,691,000
Dollars per share (in dollars per share)	$ 1.74	$ 1.34	$ 5.57	$ 6.04
Effect of dilutive securities
SARs and restricted stock (in shares)	51,000	70,000	56,000	83,000
SARs and restricted stock (in dollars per share)	$ (0.01)	$ (0.01)	$ (0.03)	$ (0.05)
Net income, Diluted EPS	$ 18,659	$ 14,406	$ 59,745	$ 64,584
Weighted-Average Shares, Diluted EPS (in shares)	10,785,000	10,776,000	10,781,000	10,774,000
Diluted EPS (in dollars per share)	$ 1.73	$ 1.33	$ 5.54	$ 5.99
Stock Appreciation Rights and Restricted Stock
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	66,445	26,768	78,963	48,801
Performance Shares
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	12,680	8,414	12,680	10,994