Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share, Basic [Abstract]
Net income attributable to common stockholders	$ 24,289	$ 18,659	$ 77,691	$ 59,745
Weighted-Average Shares (in shares)	10,766,000	10,734,000	10,756,000	10,725,000
Dollars per share (in dollars per share)	$ 2.26	$ 1.74	$ 7.22	$ 5.57
Effect of dilutive securities
SARs and restricted stock (in shares)	76,000	51,000	78,000	56,000
SARs and restricted stock (in dollars per share)	$ (0.02)	$ (0.01)	$ (0.05)	$ (0.03)
Net income, Diluted EPS	$ 24,289	$ 18,659	$ 77,691	$ 59,745
Weighted-Average Shares, Diluted EPS (in shares)	10,842,000	10,785,000	10,834,000	10,781,000
Diluted EPS (in dollars per share)	$ 2.24	$ 1.73	$ 7.17	$ 5.54
Stock Appreciation Rights and Restricted Stock
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	28,575	66,445	52,434	78,963
Performance Shares
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	18,183	12,680	18,183	12,680