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Income Taxes Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Line Items]
Income (Loss) from Continuing Operations before Income Taxes, Domestic	$ (449.0)	$ (1,096.2)	$ 322.7
Income (Loss) from Continuing Operations before Income Taxes, Foreign	629.9	(159.9)	228.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	$ 180.9	$ (1,256.1)	$ 551.5
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation
Computed tax at the U.S. federal statutory rate	21.00%	21.00%	21.00%	35.00%
State and local income taxes, net of federal income tax benefit	(7.00%)	2.60%	2.00%
Percentage depletion in excess of basis	(10.30%)	2.50%	(6.70%)
Impact of non-U.S. earnings	42.10%	5.30%	11.80%
Change in valuation allowance	(330.00%)	(3.10%)	(15.20%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Percent	0.00%	(5.00%)	0.00%
Share-based excess cost/(benefits)	0.00%	0.00%	0.70%
Other items (none in excess of 5% of computed tax)	0.00%	(5.40%)	0.40%
Effective tax rate	(319.80%)	17.90%	14.00%
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount	$ (609.0)	$ (355.6)	$ 0.7
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount		(79.6)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount		14.0
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	2.7	4.8	1.0
Refund of Future Alternative Minimum Tax Credits	(5.5)		(8.6)
Effective Income Tax Rate Reconciliation, CARES Act, Amount	(23.6)
Effect of Tax Cuts and Jobs Act [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	(774.1)	(73.4)	(945.8)
Valuation allowance	683.0	1,457.1
Alternative minimum tax credit carryforwards			100.4
Sequestration Rules, amount of AMT refunded			8.6
Undistributed earnings withholding taxes			15.0
United States Tax Cuts and Jobs Act Law
Effective Income Tax Rate Continuing Operations Tax Rate Reconciliation
Computed tax at the U.S. federal statutory rate				21.00%
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount			12.2
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount		21.2	21.2
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount		12.5
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act, Amount			12.2	$ 2.3
Effect of Tax Cuts and Jobs Act [Abstract]
TaxCutsAndJobsActOf2017IncompleteAccountingChangeInTaxRateProvisionalIncomeTaxExpenseBenefit				457.5
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act, Transition Tax on Accumulated Foreign Earnings, Amount				107.7
Reduction of Deferred Tax Liability Related to Undistributed Earnings of Non-U.S. Subsidiaries				202.6
Tax Cuts and Jobs Act, Measurement Period Adjustment, Income Tax Expense (Benefit)			9.0
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount		(30.7)
Alternative minimum tax credit carryforwards				121.5
Sequestration Rules, amount of AMT refunded				8.6
Foreign Tax Credit Carryforward
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount			30.6
Foreign and Domestic Tax Authority
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount		$ (15.9)
Foreign Tax Authority
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount	$ 582.6		$ (11.7)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Other, Percent	(35.60%)	0.00%	0.00%
Effect of Tax Cuts and Jobs Act [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	$ (32.2)	$ (9.2)
Colonsay Mine Idle
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Amount		(263.4)
Plant City Closure
Effective Income Tax Rate Reconciliation, Amounts
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Amount		(81.0)
US Branch Basket Foreign Tax Credits [Member]
Effect of Tax Cuts and Jobs Act [Abstract]
Valuation allowance			$ 156.8
Foreign Tax Credit Carryforward
Effect of Tax Cuts and Jobs Act [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount			105.8	(105.8)
Valuation allowance	249.6
Tax Credit Carryforward, Amount	628.6
Anticipatory Foreign Tax Credit Carryforward
Effect of Tax Cuts and Jobs Act [Abstract]
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount				$ (440.3)
Valuation allowance	$ 235.5	$ 224.6	$ 361.6

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