BALANCE SHEET COMPONENTS - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Materials and supplies	$ 0	$ 1,872
Prepaid expenses and other current assets	5,368	2,663
Total prepaid expenses and other current assets	$ 5,368	$ 4,535

Amount of consideration paid in advance for supplies that provide economic benefits within a future period of one year or the normal operating cycle, if longer.

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Name:	us-gaap_Supplies
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant