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Stockholders' Equity (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended			3 Months Ended
	Dec. 21, 2018	Oct. 31, 2019	Aug. 31, 2018	Sep. 30, 2017	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Aug. 06, 2018
Stock transactions
Par value (per share)					$ 0.00001			$ 0.00001
2017 Form S-3
Stock transactions
Maximum aggregate offering price				$ 200.0
2019 Form S-3
Stock transactions
Maximum aggregate offering price		$ 250.0			$ 0.0
2017 Sales Agreement
Stock transactions
Common stock sold (in shares)						0
Common Stock \| 2017 Form S-3
Stock transactions
Common stock available for sale					$ 72.0
Common Stock \| Private Placement
Stock transactions
Number of warrants exercised					0
At-the-market offering (ATM) \| 2019 Form S-3
Stock transactions
Maximum aggregate offering price		$ 75.0
At-the-market offering (ATM) \| 2017 Sales Agreement
Stock transactions
Maximum aggregate offering price				$ 50.0
Investors \| Common Stock
Stock transactions
Price (in dollars per share)									$ 8.00
Common stock sold (in shares)			2,750,000
Aggregate gross proceeds									$ 21.7
Investors \| Common Stock \| Private Placement
Stock transactions
Warrants to purchase common stock, number of shares									2,750,000
Exchange Agreement \| Exchanging Stockholders
Stock transactions
Warrants to purchase common stock, number of shares	1,000,000
Common stock exchanged for pre-funded warrants							1,000,000
Par value (per share)	$ 0.00001
Exercise Price (per share)	$ 0.00001
Duration of warrants from date of issuance (in years)	10 years
Equity method investment, ownership percentage	9.99%
Exchange Agreement \| Exchanging Stockholders \| Common Stock
Stock transactions
Number of warrants exercised					600,000
Issuance of common stock upon exercise of Exchange Warrants (in shares)					599,997
Exchange warrants unexercised					400,000
Group one \| Common Stock \| Private Placement
Stock transactions
Number of shares to be converted for each warrant									1,375,000
Exercise Price (per share)									$ 10.00
Group two \| Common Stock \| Private Placement
Stock transactions
Number of shares to be converted for each warrant									1,375,000
Exercise Price (per share)									$ 15.00