Warrants - Narrative (Details) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	May 31, 2018 $ / shares shares	May 29, 2018	Mar. 31, 2017 $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 $ / shares shares	Jan. 06, 2018 $ / shares shares	Dec. 08, 2017 shares	Dec. 07, 2017 $ / shares shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				6,436,304		6,436,304		5,452,923
Warrant exercise price (in dollars per share) | $ / shares				$ 9.83		$ 9.83		$ 11.32
Proceeds from Series A preferred stock offering allocated to warrants | $						$ 0	$ 7,623
Issued in 2017 Series A Preferred Stock Offering
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrants issued to purchase common stock (in shares)			687,468
Warrant exercise price (in dollars per share) | $ / shares			$ 12.69
Period after date of issuance to be exercised			6 months
Exercise period			5 years
Issued in December 2017 Offering - pre-funded warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				1,184,442		1,184,442		1,184,442			1,184,442
Issued in December 2017 Offering - common warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)									409,091	82,118	2,727,273
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				5,125,788		5,125,788		4,067,367
Warrant | Issued in May 2018 Private Placement - common warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)	1,528,668			1,528,668		1,528,668		0
Warrant exercise price (in dollars per share) | $ / shares	$ 2.86			$ 2.86		$ 2.86
Exercise period		5 years 6 months
Warrant | Issued in May 2018 - underwriter warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)	142,676			142,676		142,676		0
Warrant exercise price (in dollars per share) | $ / shares	$ 3.679			$ 3.679		$ 3.679
Exercise period	5 years
Warrant | Issued in 2017 Series A Preferred Stock Offering
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				687,468		687,468		687,468
Warrant exercise price (in dollars per share) | $ / shares				$ 12.69		$ 12.69
Warrant | Issued in December 2017 Offering - pre-funded warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				0		0		483,221
Warrant exercise price (in dollars per share) | $ / shares				$ 0.05		$ 0.05
Warrant | Issued in December 2017 Offering - common warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				2,679,702		2,679,702		2,809,404
Warrant exercise price (in dollars per share) | $ / shares				$ 3.85		$ 3.85		$ 3.85	$ 3.85		$ 3.85
Exercise period						5 years		5 years
Warrant | Issued in December 2017 Offering - underwriter warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				87,274		87,274		87,274
Warrant exercise price (in dollars per share) | $ / shares				$ 4.8125		$ 4.8125		$ 4.8125
Exercise period						5 years		5 years
Dividend yield
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Anticipated dividend rate				0		0
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)				1,310,516		1,310,516		1,385,556
Warrant | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-cash income (expense) | $				$ 100	$ (9,700)	$ 300	$ (9,700)
Warrant | Dividend yield
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Anticipated dividend rate				0		0		0