Stock-Based Compensation Plans - Restricted stock activity (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
The 2007 plan
Number Outstanding
Number of shares available	337,784
The 2007 plan | Restricted stock
Number Outstanding
Balance at the beginning of the period (in shares)	328,122
Shares vested	(92,730)
Balance at the end of the period (in shares)	248,392
The 2007 plan | Restricted stock | Awards Vesting Over Less Than One Year [Member]
Number Outstanding
Shares granted	13,000
Employee Stock Purchase Plan
Number Outstanding
Shares issued	5,497	6,065
Maximum percentage of cash compensation allowed to be deducted for the purchase of common stock by eligible employees	10.00%
Maximum number of shares to be purchased by employee in one calendar year	1,000
Maximum amount of shares to be purchased by employee in one calendar year (in dollars)	$ 25,000
Purchase price expressed as a percentage of the market price of the common stock on the purchase date or on the date of entry	85.00%