Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Assets
Derivative assets	$ 74.8	$ 120.0
Liabilities
Derivative liabilities	96.6	15.1
Fair value measurements on a recurring basis
Assets
Derivative assets	74.8	120.0
Investments held in rabbi trust	50.9	79.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	16.3	46.4
Investments held in rabbi trust	50.9	79.6
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	50.7	71.2
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	7.8	2.4
Investments held in rabbi trust	0.0	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	32.5	64.6
Liabilities
Derivative liabilities	96.6	15.1
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	16.3	46.4
Liabilities
Derivative liabilities	81.4	8.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	16.2	18.2
Liabilities
Derivative liabilities	15.2	6.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative assets	7.8	2.4
Fair value measurements on a recurring basis \| FTRs \| Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 2
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 3
Assets
Derivative assets	7.8	2.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	34.5	53.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative assets	0.0	0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	34.5	53.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	$ 0.0	$ 0.0

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and elected not to be offset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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