Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic EPS
Net income attributable to common stockholders	$ 29,066	$ 14,406	$ 28,304	$ 21,874	$ 19,560	$ 26,474	$ 26,340	$ 22,524	$ 93,650	$ 94,898	$ 71,300
Weighted-Average Shares (in shares)									10,696,000	10,654,000	10,594,000
Dollars per share (in dollars per share)	$ 2.72	$ 1.34	$ 2.65	$ 2.05	$ 1.84	$ 2.48	$ 2.47	$ 2.12	$ 8.76	$ 8.91	$ 6.73
Effect of dilutive securities
SARs and restricted stock (in shares)									82,000	92,000	209,000
SARs and restricted stock (in dollars per share)									$ (0.07)	$ (0.08)	$ (0.13)
Net income, Diluted EPS									$ 93,650	$ 94,898	$ 71,300
Weighted-Average Shares, Diluted EPS (in shares)									10,778,000	10,746,000	10,803,000
Diluted EPS (in dollars per share)	$ 2.70	$ 1.33	$ 2.63	$ 2.03	$ 1.82	$ 2.46	$ 2.45	$ 2.10	$ 8.69	$ 8.83	$ 6.60
Stock Appreciation Rights and Restricted Stock
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)									43,374	15,812	28,579
Performance Shares
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)									10,349	13,138	29,647