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Asset Retirement Obligations (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2019
USD ($)
years
Dec. 31, 2018
USD ($)
years
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Balance at beginning of period $ 750.2 $ 691.1
Liabilities incurred or acquired 0.0 16.3
Liabilities settled or disposed (47.7) (57.8)
Accretion expense 54.1 48.5
Revisions to estimates (4.3) 52.1
Balance at end of period 752.3 750.2
Less: Current portion (included in “Accounts payable and accrued expenses”) 98.2 63.8
Noncurrent obligation (included in “Asset retirement obligations”) 654.1 686.4
Balance at end of period — active locations 525.4 671.8
Balance at end of period — closed or inactive locations $ 226.9 $ 78.4
Life of mine 20 years or greater | years 20 20
Life of mine 3 years or less | years 3 3
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities $ 1,401.7 $ 1,317.0
Letters of credit in support of reclamation obligations or activities $ 106.1 $ 142.3
LOM 3 Years or Less    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Credit adjusted risk free interest rates 9.24% 7.61%
LOM Greater than 20 Years    
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]    
Credit adjusted risk free interest rates 12.38% 11.54%