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Operations and Significant Accounting Policies Consolidated Statement of Income (Details) $ in Millions | 9 Months Ended
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Sep. 30, 2018 USD ($)
Operating Revenue – Utility [Member] |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |
Reduction to Operating Revenue – Utility | $ 4.4
Out-of-Period Adjustment for MISO Rates [Domain] |
Quantifying Misstatement in Current Year Financial Statements [Line Items] |
Immaterial Error Correction | In the third quarter of 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 quarter and nine months ended September 30, 2018, as well as on the previous interim and annual periods in which they should have been recognized, and concluded that this adjustment is not material to any of the periods affected.

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