Subsequent Events - (Details) - USD ($)	1 Months Ended
	Feb. 28, 2018	Jan. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent events
Exercise price (in dollars per share)			$ 8.88	$ 5.28	$ 4.43
Follow-on offering
Subsequent events
Proceeds, net of underwriting discounts and commissions, but before offering costs	$ 24,800,000
Subsequent Events | Follow-on offering
Subsequent events
Number of shares sold	3,772,447
Offering price	$ 7.00
Proceeds, net of underwriting discounts and commissions, but before offering costs	$ 24,800,000
Subsequent Events | Chief Executive Officer
Subsequent events
Number of unvested stock option award modified	400,000
Exercise price (in dollars per share)	$ 7.64	$ 16.25
Incremental modification value	$ 600,000
Immediate vesting | Subsequent Events | Chief Executive Officer
Subsequent events
Incremental modification value	63,000
Remaining vesting | Subsequent Events | Chief Executive Officer
Subsequent events
Incremental modification value	$ 537,000