Operations and Significant Accounting Policies - Consolidated Statement of Income (Details)	12 Months Ended
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.

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section S99
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=112272810&loc=d3e30840-122693

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Balance Type:	na
Period Type:	duration