Warrants - Narrative (Details) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended
	Jul. 31, 2018 $ / shares shares	May 31, 2018 $ / shares shares	Dec. 31, 2017 $ / shares shares	Mar. 31, 2017 $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2018 USD ($)	Dec. 31, 2018 $ / shares shares	Mar. 31, 2018 shares	Jan. 06, 2018 shares	Dec. 08, 2017 shares	Dec. 07, 2017 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)					7,392,966		7,392,966
Warrant exercise price (in dollars per share) | $ / shares					$ 7.36		$ 7.36
Dividend yield
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Anticipated dividend rate					0
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
Issued in December 2017 Offering - common warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)			82,118						409,091	82,118	2,727,273
Issued in 2017 Series A Preferred Stock Offering
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Warrant exercise price (in dollars per share) | $ / shares				$ 12.69
Exercise period				5 years
Warrants issued to purchase common stock (in shares)				687,468
Period after date of issuance to be exercised				6 months
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)					1,205,840		1,205,840
Warrant | Dividend yield
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Anticipated dividend rate					0		0
Warrant | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-cash income (expense) | $					$ 10	$ 300
Warrant
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)					6,187,126		6,187,126
Warrant | Issued in July 2018 Private Placement - common warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)	958,152				958,152		958,152
Warrant exercise price (in dollars per share) | $ / shares	$ 2.70				$ 2.70
Exercise period	5 years 6 months
Warrant | Issued in July 2018 - underwriter warrants
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of warrants (shares)	103,186				103,186		103,186
Warrant exercise price (in dollars per share) | $ / shares	$ 3.464				$ 3.464
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
Warrant exercise price (in dollars per share) | $ / shares		$ 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
Warrant exercise price (in dollars per share) | $ / shares		$ 3.679			$ 3.679
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)			2,809,404		2,679,702		2,679,702
Warrant exercise price (in dollars per share) | $ / shares			$ 3.85		$ 3.85
Exercise period			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
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
Warrant exercise price (in dollars per share) | $ / shares					$ 12.69