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Risk Management Activities (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | May 22, 2017
Derivative [Line Items] |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 |
Interest Rate Swap |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, Notional Amount |  | $ 500.0 |  | 500.0 |  |  | $ 950.0
Interest Rate Swap Settled |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |
Derivative, Notional Amount |  | 750.0 |  | 750.0 |  |  |
Derivative, Gain (Loss) on Derivative, Net | [1] | $ 25.0 | $ 21.2 | $ 46.2 |  |  |
[1] | See Note 9, "Risk Management Activities" for additional information.

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- Definition The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 25 -Section 50 -Paragraph 1 -Subparagraph (a)(1),(2) -URI http://asc.fasb.org/extlink&oid=109977999&loc=d3e76258-113986 + Details
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