Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share, Basic [Abstract]
Net income attributable to common stockholders	$ 25,444	$ 31,687	$ 53,402	$ 41,086
Weighted-Average Shares (in shares)	10,763,000	10,730,000	10,751,000	10,721,000
Dollars per share (in dollars per share)	$ 2.36	$ 2.95	$ 4.96	$ 3.83
Effect of dilutive securities
SARs and restricted stock (in shares)	75,000	46,000	78,000	58,000
SARs and restricted stock (in dollars per share)	$ (0.01)	$ (0.01)	$ (0.03)	$ (0.02)
Net income, Diluted EPS	$ 25,444	$ 31,687	$ 53,402	$ 41,086
Weighted-Average Shares, Diluted EPS (in shares)	10,838,000	10,776,000	10,829,000	10,779,000
Diluted EPS (in dollars per share)	$ 2.35	$ 2.94	$ 4.93	$ 3.81
Stock Appreciation Rights and Restricted Stock
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	29,043	83,297	64,364	85,223
Performance Shares
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	18,183	12,680	18,183	12,680