Stockholders' Equity - Stock incentive plan (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity
Number of common stock shares purchased and retired	70,483	67,555	73,453
Amount of common stock purchased and retired	$ 74,000	$ 66,000	$ 65,000
Common stock shares reserved for issuance under the 2012 Stock Incentive Plan	2,000,000
Shares available for granting options or stock awards	1,064,926
Nonvested restricted stock
Stockholders' Equity
Vesting percentage on each anniversary of the grant date	20.00%
Service period to amortize aggregate market value of the nonvested restricted stock	6 years
Number of Shares
Nonvested at the beginning of the year (in shares)	813,000
Granted (in shares)	208,500
Vested (in shares)	(190,900)
Forfeited (in shares)	(10,200)
Nonvested at the end of the year (in shares)	820,400	813,000
Weighted Average Grant Date Fair Value
Nonvested at the beginning of the year (in dollars per share)	$ 1.45
Granted (in dollars per share)	1.05	$ 0.97	$ 0.89
Vested (in dollars per share)	2.00
Forfeited (in dollars per share)	1.30
Nonvested at the end of the year (in dollars per share)	$ 1.22	$ 1.45
Fair value of shares vested	$ 381,000	$ 477,000	$ 608,000
Compensation Expense	295,000	$ 326,000	$ 375,000
Deferred compensation cost related to nonvested restricted stock	$ 476,000
Weighted-average period to recognize deferred compensation cost related to nonvested restricted stock	3 years 6 months