Asset Retirement Obligations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance at beginning of period	$ 752.3	$ 750.2
Liabilities settled or disposed	(38.4)	(47.7)
Accretion expense	56.0	54.1
Revisions to estimates	(41.7)	(4.3)
Balance at end of period	728.2	752.3
Less: Current portion (included in “Accounts payable and accrued expenses”)	77.7	98.2
Asset retirement obligations	650.5	654.1
Balance at end of period — active locations	471.8	525.4
Balance at end of period — closed or inactive locations	256.4	226.9
Surety bonds and bank guarantees outstanding to secure reclamation obligations or activities	1,451.9	1,401.7
Letters of credit in support of reclamation obligations or activities	$ 315.0	$ 106.1
Maximum
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Life of mine	20 years
Minimum
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Life of mine	3 years
LOM 3 Years or Less
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Credit adjusted risk free interest rates	9.16%	9.24%
LOM Greater than 20 Years
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Credit adjusted risk free interest rates	12.74%	12.38%