Quarterly report pursuant to Section 13 or 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) - USD ($)
shares in Millions, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Fair value of long-term debt $ 5,300.0   $ 5,300.0   $ 4,800.0
Carrying value of long-term debt 5,000.0   5,000.0   $ 4,400.0
Carrying value of equity investment 124.9 $ 133.8 124.9 $ 133.8  
Unrealized gain on on equity investment 17.3 $ 129.9   $ 129.9  
Unrealized loss on equity investment     0.1    
Other Current Assets          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Restricted cash and cash equivalents 18.9   18.9    
Other Current Liabilities          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Restricted cash and cash equivalents $ 18.9   $ 18.9    
Stock options          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Antidilutive stock options (in shares) 0.0 0.0 0.0 0.6