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FAIR VALUE MEASUREMENT (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Long-term debts [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value $ 652.9 $ 1,064.3
Gold stream obligation [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 182.4 273.5
Cash and cash equivalent [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 103.7 216.2
Trade and other receivables [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 36.6 29.0
Investment [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 0.8 1.0
Proceeds due from income tax refunds at Mesquite [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value [1] 8.5 0.0
Financial liabilities at amortised cost, category [member] | Long-term debts [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 780.5 1,007.7
Financial liabilities at amortised cost, category [member] | Trade and other receivables [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value [2] 101.3 146.0
Financial liabilities at fair value through profit or loss, category [member] | Level 2 of fair value hierarchy [member] | Gold price option contracts [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 4.8 0.0
Financial liabilities at fair value through profit or loss, category [member] | Level 2 of fair value hierarchy [member] | Copper price option contracts [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 0.0 4.1
Financial liabilities at fair value through profit or loss, category [member] | Level 1 of fair value hierarchy [member] | Restricted share units [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 0.3 0.8
Financial liabilities at fair value through profit or loss, category [member] | Level 3 of fair value hierarchy [member] | Gold stream obligation [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 182.4 273.5
Financial liabilities at fair value through profit or loss, category [member] | Level 3 of fair value hierarchy [member] | Perfomance share units [Member]    
FINANCIAL LIABILITIES    
Financial liabilities, at fair value 0.2 1.8
Financial assets at amortised cost, category [member] | Cash and cash equivalent [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 103.7 216.2
Financial assets at amortised cost, category [member] | Trade and other receivables [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 36.6 29.0
Financial assets at fair value through profit or loss, category [member] | Level 2 of fair value hierarchy [member] | Provisionally priced contracts [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value (1.6) 4.2
Financial assets at fair value through profit or loss, category [member] | Level 2 of fair value hierarchy [member] | Gold and copper swap contracts [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 0.9 (6.1)
Financial assets at fair value through profit or loss, category [member] | Level 2 of fair value hierarchy [member] | Copper price option contracts [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 0.7 0.0
Financial assets at fair value through profit or loss, category [member] | Level 1 of fair value hierarchy [member] | Investment [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value 0.8 1.0
Financial assets at fair value through profit or loss, category [member] | Level 3 of fair value hierarchy [member] | Proceeds due from income tax refunds at Mesquite [Member]    
FINANCIAL ASSETS    
Financial assets, at fair value [1] $ 8.5 $ 0.0
[1] Proceeds due from income tax refunds at Mesquite are included in other non-current assets on the consolidated statement of financial position.
[2] Trade and other payables exclude the short-term portion of reclamation and closure cost obligations and the short-term portion of the gold stream obligation.