Income Taxes Effective tax rate (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2018
Income Tax [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ 553.5	$ 18.7	$ (16.4)
Valuation allowance	584.1	30.6
Undistributed earnings of non-U.S. subsidiaries	15.0	213.8
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount	$ 15.1	(54.2)	(28.2)
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount		$ 23.3
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount			$ (7.5)
Computed tax at the U.S. federal statutory rate of 35%	35.00%	35.00%	35.00%
Deferred Income Tax Expense (Benefit)	$ 616.6	$ (111.5)	$ 56.6
The components of earnings from consolidated companies before income taxes, and the effects of significant adjustments to tax computed at the federal statutory rate, were as follows:
United States earnings (loss)	(82.5)	(96.4)	676.0
Non-U.S. earnings	456.5	338.8	427.3
Earnings from consolidated companies before income taxes	$ 374.0	$ 242.4	$ 1,103.3
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation
State and local income taxes, net of federal income tax benefit	(0.10%)	(6.10%)	(0.50%)
Percentage depletion in excess of basis	(13.20%)	(34.40%)	(11.00%)
Impact of non-U.S. earnings	(46.90%)	(4.00%)	(13.60%)
Change in valuation allowance	148.80%	7.70%	(0.10%)
Resolution of uncertain tax positions	0.00%	(34.90%)	0.00%
Share-based excess cost/(benefits)	2.00%	2.20%	0.00%
Other items (none in excess of 5% of computed tax)	6.70%	3.90%	(0.80%)
Effective tax rate	132.30%	(30.60%)	9.00%
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	$ 6.1
Effective income tax rate reconciliation, resolution of Advanced Pricing Agreement, amount		$ (85.8)	$ (14.5)
Effective Income Tax Rate Reconciliation, Disposition of Business, Amount			(6.2)
Effective Income Tax Rate Reconciliation, Tax Contingency, State and Local, Amount			$ (18.4)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Amount	7.5	$ 8.3
Canada
Income Tax [Line Items]
Increase (Decrease) in Royalties Payable	10.4
United States
Income Tax [Line Items]
Deferred Income Tax Expense (Benefit)	14.9
Peru
Income Tax [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	6.7
United States Tax Cuts and Jobs Act Law [Member]
Income Tax [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	546.1
Valuation allowance	440.3
Alternative minimum tax credit carryforwards	121.5
Estimated impacts on deferred tax and tax expenses	457.5
Undistributed earnings of non-U.S. subsidiaries	202.6
Sequestration Rules, refundable alternative minimum tax	8.6
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount	107.7
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 2.3
Computed tax at the U.S. federal statutory rate of 35%	21.00%
Foreign Tax Credit Carryforward [Member]
Income Tax [Line Items]
Tax Credit Carryforward, Amount	$ 322.9
Foreign Tax Credit Carryforward [Member] | United States Tax Cuts and Jobs Act Law [Member]
Income Tax [Line Items]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	105.8
Valuation allowance	105.8
Alternative Minimum Tax Credit Carryforward [Member]
Income Tax [Line Items]
Tax Credit Carryforward, Amount	46.8
Alternative Minimum Tax Credit Carryforward [Member] | United States Tax Cuts and Jobs Act Law [Member]
Income Tax [Line Items]
Tax Credit Carryforward, Amount	$ 121.5
Estimate | Foreign Tax Credit Carryforward [Member] | United States Tax Cuts and Jobs Act Law [Member]
Income Tax [Line Items]
Valuation allowance				$ 440.3