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Financial Instruments and Risk Management (Contractual Commitments) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Disclosure of detailed information about financial instruments [abstract] |  |
Operating and financing leases, less than 1 year | $ 0.6 |
Operating and financing leases, 2-3 years | 0.9 |
Operating and financing leases, 4-5 years | 0.9 |
Operating and financing leases, more than 5 years | 0.5 |
Operating and financing leases, total | 2.9 |
Accounts payable and accrued liabilities, less than 1 year | 131.4 | $ 127.3
Accounts payable and accrued liabilities, 2-3 years | 0.0 |
Accounts payable and accrued liabilities, 4-5 years | 0.0 |
Accounts payable and accrued liabilities, more than 5 years | 0.0 |
Accounts payable and accrued liabilities, total | 131.4 |
Decommissioning liability, less than 1 year | 0.0 |
Decommissioning liability, 2-3 years | 0.2 |
Decommissioning liability, 4-5 years | 18.0 |
Decommissioning liability, more than 5 years | 58.4 |
Decommissioning liability, total | 76.6 |
Contract mining, less than 1 year | 64.7 |
Contract mining, 2-3 years | 81.8 |
Contract mining, 4-5 years | 42.1 |
Contract mining, more than 5 years | 25.7 |
Contract mining, total | 214.3 |
Capital commitments, less than 1 year | 30.9 |
Capital commitments, 2-3 years | 5.2 |
Capital commitments, 4-5 years | 0.0 |
Capital commitments, more than 5 years | 0.0 |
Capital commitments, total | 36.1 |
Contractual obligation, less than 1 year | 227.6 |
Contractual obligation, 2-3 years | 88.1 |
Contractual obligation, 4-5 years | 61.0 |
Contractual obligation, more than 5 years | 84.6 |
Contractual obligation, total | $ 461.3 |

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