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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.