Stock Options (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Apr. 05, 2012
Stock options under 2012 Plan
Stock option details
Shares available for future grants	232,200		800,000
Maximum exercisable period for options granted	10 years
Stock option activity
Options outstanding at the beginning of the period (in shares)	668,166
Less:
Options exercised (in shares)	25,013
Options forfeited (in shares)	15,800
Options outstanding at the end of the period (in shares)	627,353
Options fully vested and exercisable at the end of the period (in shares)	344,435
Stock Options, Weighted average exercise price
Options outstanding at the beginning, weighted average exercise price (in dollars per share)	$ 20.41
Less:
Options exercised, weighted average exercise price (in dollars per share)	17.60
Options forfeited, weighted average exercise price (in dollars per share)	18.58
Options outstanding at the end, weighted average exercise price (in dollars per share)	20.57
Options fully vested and exercisable at the end, weighted average exercise price (in dollars per share)	$ 19.47
Stock Options, Weighted average remaining contractual term (years)
Options outstanding at the end, weighted average remaining contractual term (years)	5 years 4 months 10 days
Options fully vested and exercisable at the end, weighted average remaining contractual term (years)	4 years 8 months 9 days
Stock Options, Aggregate intrinsic value
Options outstanding at the end, aggregate intrinsic value	$ 11,499,000
Options fully vested and exercisable at the end, aggregate intrinsic value	6,692,000
Stock-based compensation expense	184,800	$ 268,000
Stock-based compensation cost, unrecognized, related to non-vested options	$ 1,233,600
Stock-based compensation cost, unrecognized, related to non-vested options, weighted-average period of recognition	1 year 3 months 18 days
Incentive stock options granted to 10% shareholders
Stock option details
Maximum exercisable period for options granted	5 years