Income and Mining Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Foreign income tax expense (benefit)	$ 3.0	$ (1.4)	$ 2.1	$ 7.2
Unrecognized tax benefits	3.7		3.7		$ 4.3
Income tax penalties and interest expense			3.5		$ 4.8
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	1.5		1.5
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	$ 2.5		$ 2.5