STOCKHOLDERS' EQUITY (DEFICIT) - Equity Financing (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended		3 Months Ended						9 Months Ended		13 Months Ended
	Jun. 01, 2021	Feb. 28, 2021	Nov. 30, 2019	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Oct. 31, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Common stock, par value (in dollars per share)				$ 0.001						$ 0.001			$ 0.001
Aggregate sales price	$ 200.0
Issuance of common stock, issuance costs				$ 1.2	$ 2.0	$ 2.2	$ 0.3	$ 0.1	$ 0.1
ATM Facility
Class of Stock [Line Items]
Common stock, par value (in dollars per share)			$ 0.001
Aggregate sales price			$ 35.0
Additional shares authorized for issuance (in shares)		$ 34.5
Commission fixed rate			3.00%
Proceeds from sale of stock				$ 38.2			$ 8.2			$ 175.0	$ 16.2
Sale of stock, shares issued (in shares)				9.8			7.0			56.9	16.8
Stock price per share (in dollars per share)				$ 4.01			$ 1.21			$ 3.17	$ 0.99
Issuance of common stock, issuance costs				$ 1.2						$ 5.4
ATM Facility | Forecast
Class of Stock [Line Items]
Additional shares authorized for issuance (in shares)												$ 50.0