Variable Interest Entities (Details) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) MW	Dec. 31, 2019 USD ($)
ATC
Variable interest entities
Equity method investment, ownership interest (as a percent)	60.00%
Equity investment	$ 1,733.5	$ 1,684.7
ATC Holdco
Variable interest entities
Equity method investment, ownership interest (as a percent)	75.00%
Equity investment	$ 30.8	$ 36.1
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	$ 13.6

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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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

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.

+ References

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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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- Definition

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

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- Definition

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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