Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Assets
Derivative assets	$ 120.0	$ 17.9
Liabilities
Derivative liabilities	15.1	22.1
Fair value measurements on a recurring basis
Assets
Derivative assets	120.0	17.9
Investments held in rabbi trust	79.6	79.6
Liabilities
Derivative liabilities		22.1
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	46.4	11.7
Investments held in rabbi trust	79.6	79.6
Liabilities
Derivative liabilities		7.7
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	71.2	3.8
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities		14.4
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	2.4	2.4
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	64.6	13.7
Liabilities
Derivative liabilities	15.1	14.1
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	46.4	11.7
Liabilities
Derivative liabilities	8.4	7.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	18.2	2.0
Liabilities
Derivative liabilities	6.7	6.4
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	2.4	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	2.4	2.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	53.0	1.8
Liabilities
Derivative liabilities		1.2
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	53.0	1.8
Liabilities
Derivative liabilities		1.2
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	$ 0.0	0.0
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| Interest rate swaps
Liabilities
Derivative liabilities		6.8
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 1
Liabilities
Derivative liabilities		0.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 2
Liabilities
Derivative liabilities		6.8
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 3
Liabilities
Derivative liabilities		$ 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
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Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 55
-Paragraph 22
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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
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