Fair Value Measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Assets
Derivative asset	$ 6.1	$ 9.9
Liabilities
Derivative liability	49.0	59.0
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	1.7	2.8
Investment in exchange-traded funds	41.7	39.8
Liabilities
Derivative liability	12.6	21.4
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	3.3	3.5
Investment in exchange-traded funds	0.0	0.0
Liabilities
Derivative liability	36.4	37.6
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	1.1	3.6
Investment in exchange-traded funds	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis \| Total
Assets
Derivative asset	6.1	9.9
Investment in exchange-traded funds	41.7	39.8
Liabilities
Derivative liability	49.0	59.0
Natural gas contracts \| Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	0.6	1.6
Liabilities
Derivative liability	8.8	16.5
Natural gas contracts \| Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	1.7	1.5
Liabilities
Derivative liability	21.7	25.3
Natural gas contracts \| Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Natural gas contracts \| Fair value measurements on a recurring basis \| Total
Assets
Derivative asset	2.3	3.1
Liabilities
Derivative liability	30.5	41.8
FTRs \| Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	0.0	0.0
FTRs \| Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	0.0	0.0
FTRs \| Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	1.1	3.6
FTRs \| Fair value measurements on a recurring basis \| Total
Assets
Derivative asset	1.1	3.6
Petroleum products contracts \| Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	1.1	1.2
Liabilities
Derivative liability	3.8	4.9
Petroleum products contracts \| Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Petroleum products contracts \| Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Petroleum products contracts \| Fair value measurements on a recurring basis \| Total
Assets
Derivative asset	1.1	1.2
Liabilities
Derivative liability	3.8	4.9
Coal contracts \| Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Coal contracts \| Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	1.6	2.0
Liabilities
Derivative liability	14.7	12.3
Coal contracts \| Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Coal contracts \| Fair value measurements on a recurring basis \| Total
Assets
Derivative asset	1.6	2.0
Liabilities
Derivative liability	$ 14.7	$ 12.3

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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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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=49167202&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
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Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

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