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Derivative Financial Instruments and Hedging - Narrative (Details) € in Millions, lb in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 USD ($) lb | Dec. 31, 2016 USD ($) | Dec. 31, 2015 EUR (€) | Dec. 31, 2015 USD ($) | Dec. 31, 2017 EUR (€) | Dec. 31, 2016 EUR (€)
Derivative Instruments and Hedging Activities Disclosure [Abstract] |  |  |  |  |  |
Maximum amount of time hedged for nickel requirements | 2021 |  |  |  |  |
Percentage of estimated annual nickel requirements | 20.00% |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Pre-tax cash flow hedge gain (loss) to be reclassified within twelve months | $ (7.6) |  |  |  |  |
Nickel [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative notional amount (in pounds of nickel) | lb | 18 |  |  |  |  |
Cash Flow Hedging [Member] | Natural gas contracts [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Percentage of forecasted natural gas usage hedged for 2019 | 35.00% |  |  |  |  |
Percentage of forecasted natural gas usage hedged for 2020 | 15.00% |  |  |  |  |
Pre-tax loss on ineffective portion of hedge |  | $ 1.3 |  | $ 3.3 |  |
Percentage Of Forecasted Natural Gas Usage Hedged for 2018 | 40.00% |  |  |  |  |
Cash Flow Hedging [Member] | Foreign Exchange Forward [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative notional amount settled | € |  |  | € 211.9 |  |  |
Proceeds from hedge |  |  |  | 56.5 |  |
Pre-tax gain on ineffective portion of hedge |  |  |  | 14.3 |  |
Fair Value Hedging [Member] | Foreign Exchange Forward [Member] | Designated as Hedging Instrument [Member] | Maturity Dates Through 2017 [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Derivative notional amount | € |  |  | 139.2 |  | € 0.0 | € 43.2
Notional amount of derivatives entered during period | € |  |  | € 244.7 |  |  |
Cost of Sales [Member] | Fair Value Hedging [Member] | Foreign Exchange Forward [Member] | Designated as Hedging Instrument [Member] |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |
Gain (loss) on derivatives | $ (2.7) | $ 1.0 |  | $ 9.0 |  |

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- Definition Maximum length of time over which the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments + References No definition available. + Details
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