Stockholders' Equity - Common Stock (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 04, 2019	Nov. 30, 2018	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2019
Class of Stock
Stock issued (in shares)					785,848
Net proceeds from offering of common stock					$ 2.3
"At-The-Market" Equity Program
Class of Stock
Stock issued (in shares)					532,700
Net proceeds from offering of common stock					$ 1.6
Percentage of fixed commission expense of gross proceeds of shares sold in ATM agreement		3.00%
Lincoln Park Capital Fund, LLC (“Investor”)
Class of Stock
Value of shares investor has committed to purchase from time to time at Company’s sole discretion	$ 35.0
Stock issued (in shares)	269,749
Stock issued (per share)	$ 2.84
Common Stock
Class of Stock
Stock issued (in shares)			786,000	533,000
Common Stock | "At-The-Market" Equity Program
Class of Stock
Net proceeds from offering of common stock		$ 50.0