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Summary of Assets Held for Sale (Detail) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Assets held for sale $ 1.0 $ 31.0
Arctic Platinum [Member]    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Assets held for sale 1.0  
Discontinued Operations, Held-for-sale [Member]    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Assets held for sale 1.0 31.0
Discontinued Operations, Held-for-sale [Member] | Arctic Platinum [Member]    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Assets held for sale [1] $ 1.0 $ 31.0
[1] Refer to note 4 for details on the impairment in fiscal 2015.