Fair Value Measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets
Derivative asset	$ 12.4	$ 41.6
Liabilities
Derivative liability	11.6	2.4
Fair value measurements on a recurring basis
Assets
Derivative asset	12.4	41.6
Investment in exchange-traded funds	120.7	103.9
Liabilities
Derivative liability	11.6	2.4
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	3.0	10.3
Investment in exchange-traded funds	120.7	103.9
Liabilities
Derivative liability	7.0	0.3
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	5.0	26.2
Investment in exchange-traded funds	0.0	0.0
Liabilities
Derivative liability	4.6	2.1
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	4.4	5.1
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	5.7	34.3
Liabilities
Derivative liability	10.8	0.4
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative asset	1.8	10.1
Liabilities
Derivative liability	7.0	0.2
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative asset	3.9	24.2
Liabilities
Derivative liability	3.8	0.2
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	0.2
Liabilities
Derivative liability		0.1
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 1
Assets
Derivative asset	1.2	0.2
Liabilities
Derivative liability		0.1
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 2
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability		0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative asset	4.4	5.1
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	4.4	5.1
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative asset	1.1	2.0
Liabilities
Derivative liability	0.8	1.9
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	1.1	2.0
Liabilities
Derivative liability	0.8	1.9
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

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

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- Definition

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 investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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