STOCKHOLDERS' EQUITY - Stock-based compensation (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Amount recognized	$ 5.4	$ 2.7
Total estimated unrecognized compensation cost related to nonvested stock-based compensation arrangements	$ 43.1
Weighted average period recognized	1 year 6 months
General and administrative: other
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Amount recognized	$ 5.4	$ 2.7

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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