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Stockholders' Equity - Issuance of Common Stock and Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 13, 2018	May 09, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2017
Private Placement
ATM Program Activity
Fees and issuance costs	$ 1,900
Number of shares issued in transaction (in shares)		50,856,296
Share price (in dollars per share)		$ 1.64
Percentage of ownership after transaction	25.00%
Consideration received	$ 83,400
Private Placement \| Purchase Percentage Covenant, Threshold
ATM Program Activity
Percentage of ownership after transaction		5.00%
Private Placement \| Purchase Percentage Covenant, Maximum
ATM Program Activity
Percentage of ownership after transaction		30.00%
Sales Agreement \| Citigroup
Stockholders' equity
Common stock, amount authorized (up to)						$ 200,000
ATM Program Activity
Gross proceeds					$ 10,767
Fees and issuance costs					311
Net proceeds					$ 10,456
Shares issued (in shares)					3,802,500
NG Advantage Warrants \| NG Advantage
ATM Program Activity
Fair value of warrants outstanding			$ 0	$ 1,100
Gain (loss) from the change in fair value			$ 1,000	$ (500)	$ 0
NG Advantage Warrants \| Common stock \| NG Advantage
Stockholders' equity
Number of shares that can be purchased upon exercise of warrants (in shares)			765,106