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Financial Instruments and Risk Management (Fair Value for Assets and Liabilities) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of fair value measurement of assets [line items]    
Financial assets (liabilities) $ 3,636.5 $ 3,396.5
Financial liabilities (785.0) (701.2)
Level 1 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Fair value, net asset (liability) 43.7 22.8
Level 2 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Fair value, net asset (liability) 4.4 2.6
Fair value through profit or loss | Gold Options [Member] | Level 2 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities 0.0 (0.7)
Fair value through OCI | Currency Derivative Instruments [Member] | Level 2 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities   3.3
Fair value through OCI | Fuel Options [Member] | Level 2 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Financial liabilities   0.0
Fair value through OCI | Trading securities [member] | Level 1 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Financial assets (liabilities) 43.7 $ 22.8
Fair value through OCI | Currency Hedging Derivative Instruments [Member] | Level 2 [member] | Recurring fair value measurement [member]    
Disclosure of fair value measurement of assets [line items]    
Financial assets (liabilities) $ 4.3