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Revenue (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Revenue.    
Mineral royalties $ 173.7 $ 169.3
Mineral streams 443.3 401.6
Sale of prepaid gold 11.0 9.2
Oil & gas interests 47.0 30.1
Total revenue $ 675.0 $ 610.2