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Operations and Significant Accounting Policies - Consolidated Statement of Income (Details) | 12 Months Ended
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Dec. 31, 2018
Out-of-Period Adjustment for MISO Rates [Domain] |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |
Immaterial Error Correction | In 2018, we recognized a $4.4 million reduction in revenue for MISO rates that were billed in 2017 and are expected to be credited to customers in 2019. We have evaluated the effect of this out-of-period adjustment on the years ended December 31, 2018, and 2017, and concluded that this adjustment is not material to any of the periods affected.

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- Definition Describes nature and related effect on financial statements related to corrections of prior year errors in the current year financial statements when the effect of the prior year error was immaterial to the prior year financial statements and the current year correction is immaterial to the current year financial statements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 1.M.1) -URI http://asc.fasb.org/extlink&oid=112272810&loc=d3e30365-122693 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22644-107794 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 250 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SAB Topic 1.N.Q2) -URI http://asc.fasb.org/extlink&oid=112272810&loc=d3e30840-122693 + 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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