Summary of Significant Accounting Policies - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Basic EPS
Net Income	$ 28,304	$ 26,340	$ 50,178	$ 48,864
Weighted-Average Shares (in shares)	10,698,000	10,658,000	10,684,000	10,640,000
Dollars per share (in dollars per share)	$ 2.65	$ 2.47	$ 4.70	$ 4.59
Effect of dilutive securities
SARs and restricted stock (in shares)	72,000	84,000	90,000	102,000
SARs and restricted stock (in dollars per share)	$ (0.02)	$ (0.02)	$ (0.04)	$ (0.04)
Net income, Diluted EPS	$ 28,304	$ 26,340	$ 50,178	$ 48,864
Weighted-Average Shares, Diluted EPS (in shares)	10,770,000	10,742,000	10,774,000	10,742,000
Diluted EPS (in dollars per share)	$ 2.63	$ 2.45	$ 4.66	$ 4.55
Stock Appreciation Rights [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	62,660		59,818	56,976
Restricted Stock [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)		0		25,062
Performance Shares [Member]
Effect of dilutive securities
Antidilutive securities excluded from earnings per share (in shares)	12,284	13,138	12,284	13,138