Income and Mining Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Contingency [Line Items]
Foreign Income Tax Expense (Benefit), Continuing Operations	$ 3.6	$ 5.6
Unrecognized Tax Benefits	3.9		$ 4.3
Income tax penalties and interest expense	4.2		$ 4.8
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	1.5
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability	$ 2.5