Warrants - Narrative (Details) $ / shares in Units, $ in Millions			1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	May 31, 2018 $ / shares shares	May 29, 2018	Jul. 31, 2018 $ / shares shares	May 31, 2018 $ / shares shares	Mar. 31, 2017 $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 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)						7,497,642		7,497,642		5,452,923
Warrant exercise price (in dollars per share) | $ / shares						$ 8.83		$ 8.83		$ 11.32
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)										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
Warrants issued to purchase common stock (in shares)					687,468
Exercise period					5 years
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)						6,187,126		6,187,126		4,067,367
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		0
Warrant exercise price (in dollars per share) | $ / shares			$ 2.70			$ 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		0
Warrant exercise price (in dollars per share) | $ / shares			$ 3.464			$ 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			1,528,668
Warrant exercise price (in dollars per share) | $ / shares	$ 2.86			$ 2.86
Exercise period		5 years 6 months		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		142,676		0
Warrant exercise price (in dollars per share) | $ / shares	$ 3.679			$ 3.679		$ 3.679		$ 3.679
Exercise period	5 years
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
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
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) | $						$ 0.3	$ 5.0	$ 0.6	$ (4.7)
Warrant | Dividend yield
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Anticipated dividend rate						0		0		0