Fair Value Measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets
Derivative asset	$ 15.9	$ 12.4
Liabilities
Derivative liability	7.9	11.6
Fair value measurements on a recurring basis
Assets
Derivative asset	15.9	12.4
Investment in exchange-traded funds	65.0	120.7
Liabilities
Derivative liability	7.9	11.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	6.3	3.0
Investment in exchange-traded funds	65.0	120.7
Liabilities
Derivative liability	4.7	7.0
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	2.2	5.0
Investment in exchange-traded funds	0.0	0.0
Liabilities
Derivative liability	3.2	4.6
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	7.4	4.4
Investment in exchange-traded funds	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative asset	8.1	5.7
Liabilities
Derivative liability	5.5	10.8
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative asset	6.3	1.8
Liabilities
Derivative liability	4.7	7.0
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative asset	1.8	3.9
Liabilities
Derivative liability	0.8	3.8
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis \| Petroleum products contracts
Assets
Derivative asset		1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 1
Assets
Derivative asset		1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 2
Assets
Derivative asset		0.0
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 3
Assets
Derivative asset		0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative asset	7.4	4.4
Fair value measurements on a recurring basis \| FTRs \| Level 1
Assets
Derivative asset	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 2
Assets
Derivative asset	0.0	0.0
Fair value measurements on a recurring basis \| FTRs \| Level 3
Assets
Derivative asset	7.4	4.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative asset	0.4	1.1
Liabilities
Derivative liability	0.1	0.8
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative asset	0.4	1.1
Liabilities
Derivative liability	0.1	0.8
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	$ 0.0
Fair value measurements on a recurring basis \| Interest rate swaps
Liabilities
Derivative liability	2.3
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 1
Liabilities
Derivative liability	0.0
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 2
Liabilities
Derivative liability	2.3
Fair value measurements on a recurring basis \| Interest rate swaps \| Level 3
Liabilities
Derivative liability	$ 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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-Topic 210
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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
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