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Derivatives, Hedging Programs and Other Financial Instruments, Narrative (Details) - USD ($) | 3 Months Ended | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2017 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Derivative [Line Items] |  |  |  |  |  |  |
Debt Instrument, Term |  | 12 months |  |  |  |  |
Derivative Instruments in Hedges, at Fair Value, Net | $ 19,100,000 | $ 19,100,000 |  |  |  |  | $ 4,000,000
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months |  | 500,000 |  |  |  |  |
Margin Deposit Assets | 0 | 0 |  |  |  |  | $ 0
Customer Deposits, Current | 0 | 0 |  |  |  |  |
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net | $ 0 | $ 0 |  |  |  |  |
Scenario, Forecast | Natural Gas Member |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage |  |  | 39.00% | 71.00% | 71.00% | 72.00% |
Scenario, Forecast | Electricity Member |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, Nonmonetary Notional Amount, Percent of Required Need, Coverage |  |  | 18.00% | 55.00% | 55.00% | 54.00% |

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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994 + Details
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- Definition The current portion of money or property received from customers which is either to be returned upon satisfactory contract completion or applied to customer receivables in accordance with the terms of the contract or the understandings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Aggregate net fair value of all derivative instruments designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations. + 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 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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The percentage of expected requirements covered by the aggregate notional amount of derivative contracts expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract as a percentage of expected need. + References No definition available. + Details
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- Definition The net gain (loss) recognized in earnings during the reporting period representing the amount of the cash flow hedges' ineffectiveness. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition The amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8,17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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