VARIABLE INTEREST ENTITIES - TRANSMISSION AFFILIATES AND POWER PURCHASE AGREEMENT (Details) $ in Millions	6 Months Ended
Jun. 30, 2021 USD ($) MW	Dec. 31, 2020 USD ($)
ATC
Variable interest entities
Ownership interest (as a percent)	60.00%
Equity investment	$ 1,749.7	$ 1,733.5
ATC Holdco
Variable interest entities
Ownership interest (as a percent)	75.00%
Equity investment	$ 32.3	$ 30.8
Power purchase agreement
Variable interest entities
Firm capacity from power purchase agreement (in megawatts) \| MW	236
Minimum energy requirements over remaining term of power purchase agreement (in megawatts) \| MW	0
Remaining term of power purchase agreement (in years)	1 year
Residual guarantee associated with power purchase agreement	$ 0.0
Required capacity payments over remaining term of power purchase agreement	$ 8.9

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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Under long-term contracts with public utility districts, cooperatives or other organizations, a utility company receives a portion of the output of a production plant constructed and financed by the district or cooperative. The utility has only a nominal or no investment at all in the plant but pays a proportionate part of the plant's costs, including debt service.

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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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Firm capacity, in megawatts, from a power purchase agreement accounted for as a finance lease.

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The minimum energy requirements, in megawatts, over the remaining term of a power purchase agreement accounted for as a finance lease.

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The remaining term, in years, of a power purchase agreement accounted for as a finance lease.

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Amount of the residual guarantee associated with a power purchase agreement accounted for as a finance lease.

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