Stockholders' Equity - Stock incentive plan (Details) - $ / shares	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Stockholders Equity
Maximum number of shares authorized for grant	2,000,000
Options granted (in shares)	151,000	153,000
Number of shares available for granting options or stock awards	1,572,059
Restricted Stock
Stockholders Equity
Vesting rights percentage each year beginning on the second anniversary date of the grant	20.00%
Service period over which the aggregate market value of stock is being amortized	6 years
Number of shares purchased and retired	46,179	44,311
Average purchase price of shares purchased and retired (in dollars per share)	$ 2.27	$ 2.33