Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 17,883	$ 20,299	$ 20,660	$ 21,667	$ 21,655	$ 21,036	$ 21,767	$ 22,019	$ 80,509	$ 86,477	$ 76,907
Weighted average shares outstanding - basic and diluted (in shares)									5,115,501	5,114,084	5,112,352
Net income per common share - basic and diluted (in dollars per share)									$ 15.74	$ 16.91	$ 15.04