Fair Value Measurements - Assets and liabilities measured on a recurring basis (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets
Derivative asset	$ 13.5	$ 15.3
Liabilities
Derivative liability	46.5	12.5
Fair value measurements on a recurring basis
Assets
Investment in exchange-traded funds	49.6
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	3.0	1.1
Investment in exchange-traded funds	49.6
Liabilities
Derivative liability	17.1	11.5
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	4.7	7.2
Investment in exchange-traded funds	0.0
Liabilities
Derivative liability	24.0	1.0
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	5.8	7.0
Investment in exchange-traded funds	0.0
Liabilities
Derivative liability	5.4	0.0
Fair value measurements on a recurring basis \| Total
Assets
Derivative asset	13.5	15.3
Liabilities
Derivative liability	46.5	12.5
Fair value measurements on a recurring basis \| Natural Gas \| Level 1
Assets
Derivative asset	2.6	1.1
Liabilities
Derivative liability	15.0	11.5
Fair value measurements on a recurring basis \| Natural Gas \| Level 2
Assets
Derivative asset	3.9	3.9
Liabilities
Derivative liability	21.5	0.8
Fair value measurements on a recurring basis \| Natural Gas \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis \| Natural Gas \| Total
Assets
Derivative asset	6.5	5.0
Liabilities
Derivative liability	36.5	12.3
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	5.8	7.0
Fair value measurements on a recurring basis \| FTRs \| Total
Assets
Derivative asset	5.8	7.0
Fair value measurements on a recurring basis \| Petroleum products \| Level 1
Assets
Derivative asset	0.4
Liabilities
Derivative liability	2.1
Fair value measurements on a recurring basis \| Petroleum products \| Level 2
Assets
Derivative asset	0.0
Liabilities
Derivative liability	0.0
Fair value measurements on a recurring basis \| Petroleum products \| Level 3
Assets
Derivative asset	0.0
Liabilities
Derivative liability	0.0
Fair value measurements on a recurring basis \| Petroleum products \| Total
Assets
Derivative asset	0.4
Liabilities
Derivative liability	2.1
Fair value measurements on a recurring basis \| Coal \| Level 1
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	0.0	0.0
Fair value measurements on a recurring basis \| Coal \| Level 2
Assets
Derivative asset	0.8	3.3
Liabilities
Derivative liability	2.5	0.2
Fair value measurements on a recurring basis \| Coal \| Level 3
Assets
Derivative asset	0.0	0.0
Liabilities
Derivative liability	5.4	0.0
Fair value measurements on a recurring basis \| Coal \| Total
Assets
Derivative asset	0.8	3.3
Liabilities
Derivative liability	$ 7.9	$ 0.2

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

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