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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES- Narrative (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2021
USD ($)
Sep. 30, 2023
USD ($)
extension_period
Sep. 30, 2022
USD ($)
Finite-Lived Intangible Assets [Line Items]      
Foreign exchange gains (losses) $ (17.7) $ (1.6) $ 6.7
Gain (loss), realized (0.6) 2.3 (14.9)
Capitalized computer software costs   $ 3.7 4.3
Period of amortization of capitalized computer software costs   5 years  
Amortization of capitalized computer software costs $ 5.0 $ 3.3 7.6
Inventories of spare parts related to long term assets   35.8 35.3
Operating lease assets   54.7 58.1
Investments in marketable securities relating to deferred compensation arrangement   $ 2.6 1.8
Minimum      
Finite-Lived Intangible Assets [Line Items]      
Estimated Lives   5 years  
Maximum      
Finite-Lived Intangible Assets [Line Items]      
Estimated Lives   50 years  
Cote Blanche      
Finite-Lived Intangible Assets [Line Items]      
Number of renewal periods (in extension periods) | extension_period   2  
Lease renewal period   25 years  
Leased Mineral Interests      
Finite-Lived Intangible Assets [Line Items]      
Estimated useful life   86 years  
Owned Mineral Properties      
Finite-Lived Intangible Assets [Line Items]      
Estimated useful life   35 years  
Buildings and structures | Minimum      
Finite-Lived Intangible Assets [Line Items]      
Estimated useful life   10 years  
Buildings and structures | Maximum      
Finite-Lived Intangible Assets [Line Items]      
Estimated useful life   30 years  
Typical Maximum Life Leasehold And Building Improvements | Minimum      
Finite-Lived Intangible Assets [Line Items]      
Estimated useful life   5 years  
Typical Maximum Life Leasehold And Building Improvements | Maximum      
Finite-Lived Intangible Assets [Line Items]      
Estimated useful life   40 years  
Prepaid Expenses and Other Current Assets      
Finite-Lived Intangible Assets [Line Items]      
Assets held-for-sale   $ 33.3 $ 44.3