Description of Business - Earnings per share (Details) - $ / shares shares 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
Income per common share - Basic (in dollars per share)	$ 0.25	$ 0.30	$ 0.28	$ 0.29	$ 0.43	$ 0.29	$ 0.32	$ 0.22	$ 1.13	$ 1.25	$ 1.22
Income per common share - Diluted (in dollars per share)	$ 0.25	$ 0.30	$ 0.28	$ 0.29	$ 0.43	$ 0.29	$ 0.32	$ 0.22	$ 1.13	$ 1.25	$ 1.22
Weighted average common shares - Basic									4,299	4,503	4,634
Weighted average common shares - Diluted									4,299	4,503	4,634
As Reported
Income per common share - Basic (in dollars per share)										$ 1.31	$ 1.26
Income per common share - Diluted (in dollars per share)										$ 1.31	$ 1.25
Weighted average common shares - Basic										4,503	4,634
Weighted average common shares - Diluted										4,514	4,653