SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Basic EPS
Net income attributable to common stockholders	$ 172,838	$ 150,979	$ 111,412
Net income attributable to common stockholders (in shares)	10,829,000	10,791,000	10,758,000
Net income attributable to common stockholders (in dollars per share)	$ 15.97	$ 14.00	$ 10.36
Effect of dilutive securities
SARs and restricted stock units (in shares)	108,000	92,000	88,000
SARs and restricted stock units (in dollars per share)	$ (0.16)	$ (0.13)	$ (0.09)
Net income, diluted EPS	$ 172,838	$ 150,979	$ 111,412
Weighted-Average Shares, diluted EPS (in shares)	10,937,000	10,883,000	10,846,000
Diluted EPS (in dollars per share)	$ 15.81	$ 13.87	$ 10.27
Stock Appreciation Rights and Restricted Stock
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	32,683	32,448	39,326
Performance Shares
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	30,648	18,535	18,183