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Derivative Instruments - Derivative assets and liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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Derivative Asset |  |
Other current derivative asset | $ 11.8 | $ 38.2
Other long-term derivative assets | 0.6 | 3.4
Derivative asset | 12.4 | 41.6
Derivative Liability |  |
Other current derivative liabilities | 10.0 | 1.9
Other long-term derivative Iiabilities | 1.6 | 0.5
Derivative liability | 11.6 | 2.4
Natural gas contracts |  |
Derivative Asset |  |
Other current derivative asset | 5.6 | 31.4
Other long-term derivative assets | 0.1 | 2.9
Derivative Liability |  |
Other current derivative liabilities | 9.4 | 0.4
Other long-term derivative Iiabilities | 1.4 | 0.0
Petroleum products contracts |  |
Derivative Asset |  |
Other current derivative asset | 1.2 | 0.2
Derivative Liability |  |
Other current derivative liabilities | 0.0 | 0.1
FTRs |  |
Derivative Asset |  |
Other current derivative asset | 4.4 | 5.1
Derivative Liability |  |
Other current derivative liabilities | 0.0 | 0.0
Coal contracts |  |
Derivative Asset |  |
Other current derivative asset | 0.6 | 1.5
Other long-term derivative assets | 0.5 | 0.5
Derivative Liability |  |
Other current derivative liabilities | 0.6 | 1.4
Other long-term derivative Iiabilities | $ 0.2 | $ 0.5

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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://www.xbrl.org/2003/role/presentationRef -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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- Definition Fair value of liability arising from contracts that meet the criteria of being accounted for as derivative instruments, financial instrument or contract used to mitigate a specified risk (hedge), energy marketing activities (trading activities), expected to be settled after one year or beyond the normal operating cycle, if longer. Examples of energy marketing activities include, but are not limited to, trading of electricity, coal, natural gas and oil using financial instruments, including, but not limited to, forward contracts, options and swaps. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 + Details
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