Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic EPS
Net income attributable to common stockholders	$ 31,687	$ 28,304	$ 41,086	$ 50,178
Weighted-Average Shares (in shares)	10,730,000	10,698,000	10,721,000	10,684,000
Dollars per share (in dollars per share)	$ 2.95	$ 2.65	$ 3.83	$ 4.70
Effect of dilutive securities
SARs and restricted stock (in shares)	46,000	72,000	58,000	90,000
SARs and restricted stock (in dollars per share)	$ (0.01)	$ (0.02)	$ (0.02)	$ (0.04)
Net income, Diluted EPS	$ 31,687	$ 28,304	$ 41,086	$ 50,178
Weighted-Average Shares, Diluted EPS (in shares)	10,776,000	10,770,000	10,779,000	10,774,000
Diluted EPS (in dollars per share)	$ 2.94	$ 2.63	$ 3.81	$ 4.66
Stock Appreciation Rights and Restricted Stock
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	83,297	62,660	85,223	59,818
Performance Shares
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	12,680	12,284	12,680	12,284