Stock-Based Compensation Plan - Stock options outstanding activity (Details) - USD ($)			9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Shares [Roll Forward]
Outstanding at beginning of year (in shares)			481,381	407,832
Granted (in shares)			130,600	122,050
Exercised (in shares)			(143,092)	(41,351)
Forfeited (in shares)			(3,720)	(8,700)
Outstanding at end of period (in shares)	465,169	479,831	465,169	479,831
Options exercisable at year end (in shares)	148,223	176,292	148,223	176,292
Weighted Average Exercise Price [Roll Forward]
Outstanding at beginning of year (in dollars per share)			$ 16.52	$ 15.33
Granted (in dollars per share)			27.76	18.39
Exercised (in dollars per share)			14.40	12.18
Forfeited (in dollars per share)			15.69	16.16
Outstanding at end of period (in dollars per share)	$ 20.34	$ 16.37	20.34	16.37
Options exercisable at year end (in dollars per share)	$ 16.51	$ 14.43	$ 16.51	$ 14.43
Weighted-Average Remaining Contractual Term (in years)
Weighted-Average Remaining Contractual Life (years)	2 years 9 months 25 days	2 years 8 months 20 days	2 years 6 months	2 years 9 months 21 days
Weighted-Average Remaining Contractual Term, Granted options			4 years 5 months 1 day	4 years 4 months
Weighted Average Remaining Contractual Term, Exercised Options			11 months 12 days	1 year 8 days
Weighted-Average Remaining Contractual Term, Lapsed or Canceled			1 year 5 months 7 days	2 years 7 months 2 days
Weighted-Average Remaining Contractual Life (years)	1 year 6 months 5 days	1 year 8 months 3 days
Aggregate Intrinsic Value
Intrinsic value	$ 3,872,525	$ 3,614,016	$ 3,872,525	$ 3,614,016	$ 5,412,143	$ 2,091,196
Aggregate Intrinsic Value, options exercised			1,815,112	301,198
Intrinsic value	$ 1,801,644	$ 1,669,471	$ 1,801,644	$ 1,669,471