Variable Interest Entities (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) MW	Dec. 31, 2017 USD ($)
ATC
Variable interest entities
Equity method investment, ownership interest (as a percent)	60.00%
Equity investment	$ 1,625.3	$ 1,515.8
ATC distributions receivable		39.9
Accounts payable for services received from ATC	$ 28.2	31.2
ATC HoldCo
Variable interest entities
Equity method investment, ownership interest (as a percent)	75.00%
Equity investment	$ 40.0	$ 37.6
Purchased power 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)	3 years
Residual guarantee associated with purchased power agreement	$ 0.0
Required payments over remaining term of purchased power agreement	$ 56.7

Amount for accounts payable to related parties. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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Carrying amount as of the balance sheet date of dividends declared but not received.

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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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Firm capacity, in megawatts, from a purchased power agreement accounted for a capital lease.

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

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

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

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