Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
Derivative assets	$ 17.9	$ 6.9
Liabilities
Derivative liabilities	22.1	28.9
Fair value measurements on a recurring basis
Assets
Derivative assets	17.9	6.9
Investments held in rabbi trust	79.6	85.3
Liabilities
Derivative liabilities	22.1	28.9
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative assets	11.7	1.4
Investments held in rabbi trust	79.6	85.3
Liabilities
Derivative liabilities	7.7	21.4
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative assets	3.8	2.4
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	14.4	7.5
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative assets	2.4	3.1
Investments held in rabbi trust	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative assets	13.7	3.4
Liabilities
Derivative liabilities	14.1	22.7
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative assets	11.7	1.4
Liabilities
Derivative liabilities	7.7	21.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative assets	2.0	2.0
Liabilities
Derivative liabilities	6.4	1.3
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	3.1
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	3.1
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative assets	1.8	0.4
Liabilities
Derivative liabilities	1.2	0.2
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative assets	1.8	0.4
Liabilities
Derivative liabilities	1.2	0.2
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative assets	0.0	0.0
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| Interest rate swaps
Liabilities
Derivative liabilities	6.8	6.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 1
Liabilities
Derivative liabilities	0.0	0.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 2
Liabilities
Derivative liabilities	6.8	6.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 3
Liabilities
Derivative liabilities	$ 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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-Publisher FASB
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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
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