Income and Mining Taxes - Narrative (Details) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 21, 2009 oz
Income Tax Contingency [Line Items]
Valuation allowance				$ 40.8
Gross unrecognized benefits			4.3	5.1	1.2
Income tax penalties and interest expense			$ 4.8	$ 5.5	$ 0.7
Minimum
Income Tax Contingency [Line Items]
Unrecognized income tax liability			1.5
Maximum
Income Tax Contingency [Line Items]
Unrecognized income tax liability			$ 2.5
Palmarejo gold production royalty
Income Tax Contingency [Line Items]
Payment made for gold on the end of royalty obligation | oz						4,167
Silver and gold options
Income Tax Contingency [Line Items]
Net derivative gain (loss)				$ 1.6	$ 1.3
Franco-Nevada warrant
Income Tax Contingency [Line Items]
Market-to-market adjustment	$ 5.9	$ 17.0