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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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STOCKHOLDERS' EQUITY - Stock-based compensation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Stock-based compensation expense	$ 8.4	$ 3.7	$ 13.8	$ 6.4
Total estimated unrecognized compensation cost related to nonvested stock-based compensation arrangements	44.6		$ 44.6
Weighted average period recognized			1 year 4 months 24 days
Board of Director
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Stock-based compensation expense			$ 0.9	0.5
Restricted stock unit
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Stock-based compensation expense	3.5	3.0	5.7	5.0
Performance stock unit expense
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Stock-based compensation expense	3.4		4.2
Special Performance Stock Unit
Components Of Recorded And Unrecognized Stock Based Compensation Expense [Rollforward]
Stock-based compensation expense	$ 1.5	$ 0.7	$ 3.0	$ 0.9