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Fair Value Measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Derivative asset | $ 15.9 | $ 12.4
Liabilities |  |
Derivative liability | 7.9 | 11.6
Fair value measurements on a recurring basis |  |
Assets |  |
Derivative asset | 15.9 | 12.4
Investment in exchange-traded funds | 65.0 | 120.7
Liabilities |  |
Derivative liability | 7.9 | 11.6
Fair value measurements on a recurring basis | Level 1 |  |
Assets |  |
Derivative asset | 6.3 | 3.0
Investment in exchange-traded funds | 65.0 | 120.7
Liabilities |  |
Derivative liability | 4.7 | 7.0
Fair value measurements on a recurring basis | Level 2 |  |
Assets |  |
Derivative asset | 2.2 | 5.0
Investment in exchange-traded funds | 0.0 | 0.0
Liabilities |  |
Derivative liability | 3.2 | 4.6
Fair value measurements on a recurring basis | Level 3 |  |
Assets |  |
Derivative asset | 7.4 | 4.4
Investment in exchange-traded funds | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Natural gas contracts |  |
Assets |  |
Derivative asset | 8.1 | 5.7
Liabilities |  |
Derivative liability | 5.5 | 10.8
Fair value measurements on a recurring basis | Natural gas contracts | Level 1 |  |
Assets |  |
Derivative asset | 6.3 | 1.8
Liabilities |  |
Derivative liability | 4.7 | 7.0
Fair value measurements on a recurring basis | Natural gas contracts | Level 2 |  |
Assets |  |
Derivative asset | 1.8 | 3.9
Liabilities |  |
Derivative liability | 0.8 | 3.8
Fair value measurements on a recurring basis | Natural gas contracts | Level 3 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Petroleum products contracts |  |
Assets |  |
Derivative asset |  | 1.2
Fair value measurements on a recurring basis | Petroleum products contracts | Level 1 |  |
Assets |  |
Derivative asset |  | 1.2
Fair value measurements on a recurring basis | Petroleum products contracts | Level 2 |  |
Assets |  |
Derivative asset |  | 0.0
Fair value measurements on a recurring basis | Petroleum products contracts | Level 3 |  |
Assets |  |
Derivative asset |  | 0.0
Fair value measurements on a recurring basis | FTRs |  |
Assets |  |
Derivative asset | 7.4 | 4.4
Fair value measurements on a recurring basis | FTRs | Level 1 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs | Level 2 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Fair value measurements on a recurring basis | FTRs | Level 3 |  |
Assets |  |
Derivative asset | 7.4 | 4.4
Fair value measurements on a recurring basis | Coal contracts |  |
Assets |  |
Derivative asset | 0.4 | 1.1
Liabilities |  |
Derivative liability | 0.1 | 0.8
Fair value measurements on a recurring basis | Coal contracts | Level 1 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | 0.0
Fair value measurements on a recurring basis | Coal contracts | Level 2 |  |
Assets |  |
Derivative asset | 0.4 | 1.1
Liabilities |  |
Derivative liability | 0.1 | 0.8
Fair value measurements on a recurring basis | Coal contracts | Level 3 |  |
Assets |  |
Derivative asset | 0.0 | 0.0
Liabilities |  |
Derivative liability | 0.0 | $ 0.0
Fair value measurements on a recurring basis | Interest rate swaps |  |
Liabilities |  |
Derivative liability | 2.3 |
Fair value measurements on a recurring basis | Interest rate swaps | Level 1 |  |
Liabilities |  |
Derivative liability | 0.0 |
Fair value measurements on a recurring basis | Interest rate swaps | Level 2 |  |
Liabilities |  |
Derivative liability | 2.3 |
Fair value measurements on a recurring basis | Interest rate swaps | Level 3 |  |
Liabilities |  |
Derivative liability | $ 0.0 |

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- Definition Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 + Details
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