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Finance Lease Liabilities - Summary of Finance Lease Liabilities (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of finance lease liabilities [line items]      
Interest expense $ (0.2) $ 0.0 $ 0.0
Balance at the end of the year 111.5    
Current portion of finance lease liability 8.5 0.0  
Non-current portion of finance lease liability 80.1 0.0  
Gruyere power purchase agreement [member]      
Disclosure of finance lease liabilities [line items]      
Balance at the beginning of the year 0.0 0.0  
Additions1 96.2 0.0  
Interest expense 0.2 0.0  
Repayments (2.5) 0.0  
Translation adjustment (5.3) 0.0  
Balance at the end of the year 88.6 0.0 $ 0.0
Current portion of finance lease liability (8.5) 0.0  
Non-current portion of finance lease liability 80.1 0.0  
Future minimum lease payments 111.5 0.0  
Interest 22.9 0.0  
Present value of minimum lease payments 88.6 0.0  
Within one year [member]      
Disclosure of finance lease liabilities [line items]      
Balance at the end of the year 11.6    
Within one year [member] | Gruyere power purchase agreement [member]      
Disclosure of finance lease liabilities [line items]      
Future minimum lease payments 11.6 0.0  
Interest 3.1 0.0  
Present value of minimum lease payments 8.5 0.0  
Later than one years and not later than five years [member] | Gruyere power purchase agreement [member]      
Disclosure of finance lease liabilities [line items]      
Future minimum lease payments 41.5 0.0  
Interest 11.5 0.0  
Present value of minimum lease payments 30.0 0.0  
Later than five years [member]      
Disclosure of finance lease liabilities [line items]      
Balance at the end of the year 58.4    
Later than five years [member] | Gruyere power purchase agreement [member]      
Disclosure of finance lease liabilities [line items]      
Future minimum lease payments 58.4 0.0  
Interest 8.3 0.0  
Present value of minimum lease payments $ 50.1 $ 0.0