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Coal Trading (Details 1) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Changes in the Company's recurring Level 3 net financial assets |  |  |  |
Beginning of year |  | $ 281.2 |  |
Total gains (realized/unrealized): |  |  |  |
End of year |  | 295.1 | $ 281.2 |
Coal Trading (Textuals) [Abstract] |  |  |  |
Fair value hierarchy transfers from Level 1 to Level 2 |  | 0.0 | 0.0 |
Fair value hierarchy transfers from Level 2 to Level 1 |  | 0.0 | 0.0 |
Coal Trading [Member] |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |
Asset Transfers Into Level 3 |  | 5.3 | (4.4) | $ 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 |  | (0.4) | 0.0 | 0.0
Changes in the Company's recurring Level 3 net financial assets |  |  |  |
Beginning of year |  | (15.6) | 2.1 | 2.1
Total gains (realized/unrealized): |  |  |  |
Included in earnings |  | (2.4) | (10.1) | 6.7
Purchases |  | 0.0 | (0.5) | 0.0
Sales |  | 0.0 | (0.1) | 0.0
Settlements |  | 12.0 | (2.6) | (6.7)
End of year |  | (1.1) | (15.6) | 2.1
Changes in unrealized (losses) gains | [1] | 0.0 | (6.2) | 2.1
Coal Trading (Textuals) [Abstract] |  |  |  |
Cash flow hedge derivative instrument assets at fair value |  | 0.0 |  |
Fair value hierarchy transfers from Level 1 to Level 2 |  | 0.0 | 0.0 | 0.0
Fair value hierarchy transfers from Level 2 to Level 1 |  | 0.0 | 0.0 | 0.0
Fair Value hierarchy transfers out of Level 3 |  |  | 0.0 | $ 0.0
Fair Value, Measurements, Recurring [Member] | Coal Trading [Member] |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis |  |  |  |
Commodity futures, swaps and options |  | (0.1) | 3.3 |
Physical commodity purchase/sale contracts |  | (0.4) | 4.6 |
Total net financial assets (liabilities) |  | (0.5) | 7.9 |
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Coal Trading [Member] |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis |  |  |  |
Commodity futures, swaps and options |  | 0.0 | 0.0 |
Physical commodity purchase/sale contracts |  | 0.0 | 0.0 |
Total net financial assets (liabilities) |  | 0.0 | 0.0 |
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Coal Trading [Member] |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis |  |  |  |
Commodity futures, swaps and options |  | (0.1) | 3.3 |
Physical commodity purchase/sale contracts |  | 0.7 | 20.2 |
Total net financial assets (liabilities) |  | 0.6 | 23.5 |
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Coal Trading [Member] |  |  |  |
Fair value coal trading net assets (liabilities) measured on recurring basis |  |  |  |
Commodity futures, swaps and options |  | 0.0 | 0.0 |
Physical commodity purchase/sale contracts |  | (1.1) | (15.6) |
Total net financial assets (liabilities) |  | $ (1.1) | $ (15.6) |
Minimum [Member] | Coal Trading [Member] |  |  |  |
Coal Trading (Textuals) [Abstract] |  |  |  |
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation |  | 2.00% |  |
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation |  | 26.00% |  |
Maximum [Member] | Coal Trading [Member] |  |  |  |
Coal Trading (Textuals) [Abstract] |  |  |  |
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation |  | 2.00% |  |
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation |  | 26.00% |  |
Weighted Average [Member] | Coal Trading [Member] |  |  |  |
Coal Trading (Textuals) [Abstract] |  |  |  |
Quality adjustment Level 3 unobservable inputs as percentage of overall valuation |  | 2.00% |  |
Credit and non-performance risk, level 3 unobservable input as a percentage of overall valuation |  | 26.00% |  |
[1] | Within the consolidated statements of operations and consolidated statements of comprehensive income for the periods presented, unrealized gains and losses from Level 3 items are combined with unrealized gains and losses on positions classified in Level 1 or 2, as well as other positions that have been realized during the applicable periods.

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