Stockholders' Equity - Warrants (Details) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Number of Warrants
Balance of warrants outstanding at the beginning of the period (in shares)	3,649,341
Granted (in shares)	1,408,510
Exercised (in shares)	(653,666)
Balance of warrants outstanding at the end of the period (in shares)	4,404,185	3,649,341
Weighted Average Exercise Price Per Share
Weighted average exercise price of warrants at the beginning of the period (in USD per share)	$ 8.91
Granted (in USD per share)	3.50
Exercised (in USD per share)	5.96
Weighted average exercise price of warrants at the end of the period (in USD per share)	$ 7.62	$ 8.91
Term
Weighted-Average Remaining Contractual Term	4 years 3 months 7 days	4 years 4 months 21 days