Accounting for Share-Based Payments - Activity (Details) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Number of Options
Forfeited (in shares)	0
Share-based payments
Total fair value of shares vested during the period	$ 1,000,000
Stock option
Number of Options
Balance outstanding at the beginning of the period (in shares)	852,648
Granted (in shares)	0
Forfeited (in shares)	(417)
Balance outstanding at the end of the period (in shares)	852,232		852,648
Vested awards and those expected to vest at the end of the period (in shares)	842,220
Vested and exercisable at the end of the period (in shares)	637,501
Weighted Average Exercise Price Per Share
Balance outstanding at the beginning of the period (in dollars per share)	$ 11.87
Granted (in dollars per share)	0.00
Exercised (in dollars per share)	0.00
Forfeited (in dollars per share)	14.16
Balance outstanding at the end of the period (in dollars per share)	11.89		$ 11.87
Vested awards and those expected to vest at the end of the period (in dollars per share)	11.85
Vested and exercisable at the end of the period (in dollars per share)	$ 11.89
Intrinsic Value
Balance outstanding at the beginning of the period (in dollars)	$ 47,667
Balance outstanding at the end of the period (in dollars)	23,833		$ 47,667
Vested awards and those expected to vest at the end of the period (in dollars)	10,010
Vested and exercisable at the end of the period (in dollars)	$ 10,010
Weighted Average Remaining Contractual Term (in years)
Balance outstanding term (in years)	6 years 6 months 7 days		7 years
Vested awards and those expected to vest at the end of the period (in years)	6 years 6 months 4 days
Vested and exercisable at the end of the period (in years)	6 years 6 months 7 days
Share-based payments
Weighted average grant date fair value (in dollars per share)	$ 0.00	$ 7.83
Unrecognized compensation cost related to non-vested stock options outstanding
Unrecognized compensation cost related to non-vested stock (in dollars)	$ 700,000
Weighted average remaining vesting period over which unrecognized compensation is expected to be recognized	2 years 2 months 12 days
Stock option | Exercise price range $0.88-$35.04
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)	$ 0.88		$ 0.88
Exercise price, high end of the range (in dollars per share)	35.04		$ 35.04
Stock option | Exercise price range $0.00-$0.00
Exercise Price Per Share
Exercise price, low end of the range (in dollars per share)	0.00
Exercise price, high end of the range (in dollars per share)	0.00
Stock option | Exercise price of $14.16
Exercise Price Per Share
Exercise price per share	$ 14.16
Non-employee stock options
Share-based payments
Options outstanding (in dollars)	$ 200,000
Non-employee stock options | Maximum
Share-based payments
Unvested options subject to remeasurement (in dollars)	$ 100,000