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Derivative Financial Instruments and Hedging - Schedule of Derivative Instruments Gain (Loss) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Recognized in OCI on Derivatives |  | $ (6.4) | $ 14.3 | $ 21.2
Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Recognized in OCI on Derivatives |  | (4.9) | 11.1 |
Cost of Sales and Interest Expense [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) | [1] | 8.9 |  |
Cost of Sales [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) | [1] |  | 4.5 |
Nickel and other raw material contracts [Member] | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Recognized in OCI on Derivatives |  | (6.4) | 14.1 |
Nickel and other raw material contracts [Member] | Cost of Sales [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) | [1] | 7.7 | (2.1) |
Natural gas contracts [Member] | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Recognized in OCI on Derivatives |  | 1.5 | (2.8) |
Natural gas contracts [Member] | Cost of Sales [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) | [1] | 0.4 | (3.3) |
Foreign Exchange Contract [Member] | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Recognized in OCI on Derivatives |  | 0.5 | (0.2) |
Foreign Exchange Contract [Member] | Cost of Sales [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) | [1] | 1.0 | 9.9 |
Interest Rate Swap [Member] | Other Comprehensive Income (Loss) [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Recognized in OCI on Derivatives |  | (0.5) | 0.0 |
Interest Rate Swap [Member] | Interest Expense [Member] | Cash Flow Hedging [Member] |  |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |  |
Amount of Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion) | [1] | $ (0.2) | $ 0.0 |
[1] | The gains (losses) reclassified from accumulated OCI into income related to the derivatives, with the exception of the interest rate swap, are presented in cost of sales in the same period or periods in which the hedged item affects earnings. The gains (losses) reclassified from accumulated OCI into income on the interest rate swap are presented in interest expense in the same period as the interest expense on the Term Loan is recognized in earnings.

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