Number of Jointly Owned Electric Generation Plants

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Number of Jointly Owned Major Transmission Lines

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Percentage of the reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of interest capitalized during the period.

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Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

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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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The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

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Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) segment plant	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Number of Operating Segments \| segment	3
Number of jointly owned electric generation plants \| plant	2
Number of jointly owned major transmission lines \| plant	5
Electric Plant in Service
Property, Plant and Equipment [Line Items]
Interest costs capitalized \| $	$ 2.1	$ 1.7	$ 1.2
OTP \| Lignite Sales Agreement \| Coyote Creek Mining Company, L.L.C. (CCMC)
Property, Plant and Equipment [Line Items]
Variable interest entity, reporting entity involvement, maximum loss exposure, amount \| $	$ 50.0
Variable interest entity, reporting entity involvement, maximum loss exposure, percentage	35.00%
Minimum
Property, Plant and Equipment [Line Items]
Intangible asset, useful life	15 years
Operating lease, term	1 year
Maximum
Property, Plant and Equipment [Line Items]
Intangible asset, useful life	20 years
Operating lease, term	10 years