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Prepaid expenses and other current assets (Details) - USD ($)
$ in Millions
Jun. 30, 2019
Dec. 31, 2018
Prepaid expenses and other current assets.    
Gold bullion $ 26.2 $ 27.8
Prepaid expenses 10.8 5.4
Debt issue costs 0.1 0.1
Prepaid expenses and other $ 37.1 $ 33.3