Variable Interest Entities (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)
ATC
Variable interest entities
Equity method investment, ownership interest (as a percent)	60.00%
Equity investment	$ 1,684.7	$ 1,625.3
ATC HoldCo
Variable interest entities
Equity method investment, ownership interest (as a percent)	75.00%
Equity investment	$ 36.1	$ 40.0
Power purchase agreement
Variable interest entities
Firm capacity from purchased power agreement (in megawatts) \| MW	236
Minimum energy requirements over remaining term of purchased power agreement (in megawatts) \| MW	0
Remaining term of purchased power agreement (in years)	2 years
Residual guarantee associated with purchased power agreement	$ 0.0
Required payments over remaining term of purchased power agreement	$ 22.4

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-Paragraph 1
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- Definition

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 purchased power agreement accounted for as a finance lease.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

The remaining term, in years, of a purchased power agreement accounted for as a capital lease.

+ References

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- Definition

Amount of the residual guarantee associated with a purchased power agreement accounted for as a capital lease.

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No definition available.

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