Carrying amount as of the balance sheet date of obligations due all related parties. For classified balance sheets, represents the current portion of such liabilities (due within one year or within the normal operating cycle if longer).

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RELATED PARTY TRANSACTIONS - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Jul. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
2021 Convertible Notes \| Amazon \| Senior Notes
Related Party Transaction [Line Items]
Fair value (carrying amount)				$ 490.0
2021 Convertible Notes \| Ford \| Senior Notes
Related Party Transaction [Line Items]
Fair value (carrying amount)				415.0
2021 Convertible Notes \| T Rowe Price \| Senior Notes
Related Party Transaction [Line Items]
Fair value (carrying amount)				$ 400.0
Investor
Related Party Transaction [Line Items]
Fair value of warrants						$ 11.0
Affiliated Entity
Related Party Transaction [Line Items]
Accrued liabilities	$ 8.0		$ 8.0		$ 27.0
Affiliated Entity \| Research and development
Related Party Transaction [Line Items]
Expenses	$ 16.0	$ 30.0	$ 49.0