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Coal Trading (Details) (Coal Trading [Member], USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2014
Jun. 30, 2013
Jun. 30, 2014
Jun. 30, 2013
Dec. 31, 2013
Trading Revenues by Type of Instrument [Line Items]          
Trading revenues $ 10.3 $ (3.4) $ 31.3 $ 22.5  
Coal trading derivative instruments and balance sheet offsetting disclosures:          
Assets from coal trading activities, gross amounts of recognized assets 430.2   430.2   418.8
Liabilities from coal trading activities, gross amounts of recognized liabilities (359.8)   (359.8)   (332.7)
Net coal trading assets (liabilities), gross amounts of recognized assets (liabilities) 70.4   70.4   86.1
Gross amounts of coal trading liabilities offset against associated coal trading assets (345.4)   (345.4)   (324.7)
Gross amounts of coal trading assets offset against associated coal trading liabilities 345.4   345.4   324.7
Net coal trading (liabilities) assets offset against associated assets (liabilities) 0   0   0
Variation margin held offset against assets from coal trading activities (34.0)   (34.0)   (58.0)
Variation margin posted offset against liabilities from coal trading activities 0   0   1.9
Net variation margin held (34.0) [1]   (34.0) [1]   (56.1) [1]
Assets from coal trading activities, net 50.8   50.8   36.1
Liabilities from coal trading activities, net (14.4)   (14.4)   (6.1)
Net assets from coal trading activities 36.4   36.4   30.0
Commodity futures, swaps and options [Member]
         
Trading Revenues by Type of Instrument [Line Items]          
Trading revenues 22.7 34.9 58.3 85.9  
Physical commodity purchase / sale contracts [Member]
         
Trading Revenues by Type of Instrument [Line Items]          
Trading revenues (12.4) (38.3) (27.0) (63.4)  
Cash Flow Hedging [Member]
         
Coal trading derivative instruments and balance sheet offsetting disclosures:          
Net variation margin held $ 15.0   $ 15.0   $ 42.0
[1] Approximately $15 million and $42 million of the net variation margin held at June 30, 2014 and December 31, 2013, respectively, related to cash flow hedges.