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Derivatives, Hedging Programs and Other Financial Instruments, Fair Value Hierarchy Table (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions | Dec. 31, 2019 | Dec. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | $ 264.3 | $ 125.6
Derivative Asset | (4.0) | (4.3)
Derivative Liability | (11.2) | (15.9)
Other Assets, Fair Value Disclosure | 343.0 | 162.3
Debt Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 78.7 | 36.7
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 28.2 | 22.9
Other Assets, Fair Value Disclosure | 28.2 | 22.9
Level 1 | Debt Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0.0 | 0.0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 236.1 | 102.7
Derivative Asset | (4.0) | (4.3)
Derivative Liability | (11.2) | (15.9)
Other Assets, Fair Value Disclosure | 314.8 | 139.4
Level 2 | Debt Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 78.7 | 36.7
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Cash and cash equivalents | 0.0 | 0.0
Other Assets, Fair Value Disclosure | 0.0 | 0.0
Level 3 | Debt Securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Available-for-sale Securities | 0.0 | 0.0
Designated as Hedging Instrument | Level 2 | Aluminum Member | Long |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | (1.0) | (0.1)
Derivative Liability | (4.1) | (13.2)
Designated as Hedging Instrument | Level 2 | Aluminum Member | Short |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 0.0 | (0.1)
Derivative Liability | 0.0 | 0.0
Designated as Hedging Instrument | Level 2 | Midwest premium swap contracts | Long |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 0.0 | (3.2)
Derivative Liability | (1.2) | (0.5)
Designated as Hedging Instrument | Level 2 | Natural Gas Member | Long |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | 0.0 | (0.2)
Derivative Liability | (2.8) | (0.5)
Designated as Hedging Instrument | Level 2 | Electricity [Member] | Long |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | (2.6) | (0.7)
Derivative Liability | (1.6) | 0.0
Designated as Hedging Instrument | Level 2 | Alloy Metal Hedge | Long |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative Asset | (0.4) | 0.0
Derivative Liability | $ (1.5) | $ (1.7)

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