Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share
Net income	$ 21,667	$ 22,019
Weighted average shares outstanding - basic and diluted (in shares)	5,114,701	5,113,077
Net income per common share (in dollars per share)	$ 4.24	$ 4.31