Warrants - Liability-Classified Warrants Outstanding (Details) - $ / shares	Jun. 30, 2019	Dec. 31, 2018	Jul. 31, 2018	May 31, 2018	Jan. 06, 2018	Dec. 31, 2017	Dec. 08, 2017	Dec. 07, 2017	Mar. 31, 2017
Warrants
Warrants issued to purchase common stock (shares)	7,392,966	7,392,966
Warrant exercise price (in dollars per share)	$ 7.36	$ 7.36
Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrant exercise price (in dollars per share)									$ 12.69
Issued in December 2017 Offering - common warrants
Warrants
Warrants issued to purchase common stock (shares)					409,091	82,118	82,118	2,727,273
Warrant
Warrants
Warrants issued to purchase common stock (shares)	1,205,840	1,205,840
Warrant | Issued with September 2016 Convertible Notes
Warrants
Warrants issued to purchase common stock (shares)	1,205,840	1,205,840
Warrant exercise price (in dollars per share)	$ 22.50
Warrant
Warrants
Warrants issued to purchase common stock (shares)	6,187,126	6,187,126
Warrant | Issued in 2017 Series A Preferred Stock Offering
Warrants
Warrants issued to purchase common stock (shares)	687,468	687,468
Warrant exercise price (in dollars per share)	$ 12.69
Warrant | Issued in December 2017 Offering - common warrants
Warrants
Warrants issued to purchase common stock (shares)	2,679,702	2,679,702				2,809,404
Warrant exercise price (in dollars per share)	$ 3.85					$ 3.85
Warrant | Issued in December 2017 Offering - underwriter warrants
Warrants
Warrants issued to purchase common stock (shares)	87,274	87,274				87,274
Warrant exercise price (in dollars per share)	$ 4.8125					$ 4.8125
Warrant | Issued in May 2018 Private Placement - common warrants
Warrants
Warrants issued to purchase common stock (shares)	1,528,668	1,528,668
Warrant exercise price (in dollars per share)	$ 2.86
Warrant | Issued in May 2018 - underwriter warrants
Warrants
Warrants issued to purchase common stock (shares)	142,676	142,676		142,676
Warrant exercise price (in dollars per share)	$ 3.679			$ 3.679
Warrant | Issued in July 2018 Private Placement - common warrants
Warrants
Warrants issued to purchase common stock (shares)	958,152	958,152	958,152
Warrant exercise price (in dollars per share)	$ 2.70		$ 2.70
Warrant | Issued in July 2018 - underwriter warrants
Warrants
Warrants issued to purchase common stock (shares)	103,186	103,186	103,186
Warrant exercise price (in dollars per share)	$ 3.464		$ 3.464