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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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