Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share
Income from continuing operations									$ 86,477	$ 76,907	$ 67,396
Income from discontinued operations									0	0	529
Net income - basic and diluted	$ 21,655	$ 21,036	$ 21,767	$ 22,019	$ 23,572	$ 18,172	$ 17,341	$ 17,822	$ 86,477	$ 76,907	$ 67,925
Weighted average shares outstanding - basic and diluted (in shares)									5,114,084	5,112,352	5,110,628
Income from continuing operations (in dollars per share)									$ 16.91	$ 15.04	$ 13.19
Income from discontinued operations (in dollars per share)									0	0	0.1
Net income per common share- basic and diluted (in dollars per share)									$ 16.91	$ 15.04	$ 13.29
Earnings Per Share Potentially Dilutive Securities									no	no	no