Income and Mining Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Foreign Income Tax Expense (Benefit), Continuing Operations	$ 4.5		$ 3.0	$ 0.9	$ 8.6
Unrecognized Tax Benefits	4.0			4.0		$ 4.3
Income tax penalties and interest expense		$ 3.7				$ 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