Stockholders' Equity - Narrative (Details)					1 Months Ended	3 Months Ended
	Aug. 10, 2017 USD ($)	Jun. 27, 2017 USD ($) $ / shares shares	Jul. 25, 2016 USD ($) $ / shares shares	Jun. 21, 2016 USD ($) $ / shares shares	Jul. 25, 2016 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 $ / shares shares	Jun. 27, 2016 shares
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)						22,349,840
Common stock, par value (in dollars per share) | $ / shares						$ 0.0001	$ 0.0001
Common stock issued for convertible preferred stock (in shares)								10,126,118
Common stock, shares authorized (in shares)						200,000,000	200,000,000
Number of votes per share that common stockholders are entitled to						1
Dividends declared or paid on common stock | $						$ 0
Common stock, shares outstanding (in shares)						22,349,840	22,343,254
At-The-Market Offering
Subsidiary, Sale of Stock [Line Items]
Public securities offering, amount authorized | $	$ 200,000,000.0
Public stock offering, amount authorized | $						$ 50,000,000
Sale of stock, number of shares issued in transaction (in shares)						0
2017 PIPE
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		2,750,000
Common stock, par value (in dollars per share) | $ / shares		$ 0.0001
Share price (in dollars per share) | $ / shares		$ 16.00
Proceeds from issuance of common stock | $		$ 47,100,000
Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares		$ 17.71
Exercise of common warrants
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		79,130
Exercise price (in dollars per share) | $ / shares		$ 17.71
Expiration period of warrants		5 years
Share price (in dollars per share) | $ / shares		$ 0.125
IPO
Subsidiary, Sale of Stock [Line Items]
Sale of stock, number of shares issued in transaction (in shares)			289,633	5,000,000
Share price (in dollars per share) | $ / shares				$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions | $				$ 60,800,000	$ 64,500,000
Over-Allotment Option
Subsidiary, Sale of Stock [Line Items]
Share price (in dollars per share) | $ / shares			$ 14.00		$ 14.00
Proceeds from Initial Public Offering, net of underwriters' discounts and commissions | $			$ 3,700,000
Common stock | Springer Purchase Agreement
Subsidiary, Sale of Stock [Line Items]
Stock issued (in shares)		338,791