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Operations and Significant Accounting Policies Consolidated Statement of Income (Details) $ in Millions	9 Months Ended
	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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- 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.

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Amount of each individual error being corrected in the correcting adjustment.

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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.

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No definition available.

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