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Derivative Financial Instruments (Details) (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
Derivative financial instruments          
Unrealized mark-to-market (losses) gains     $ (7.6) $ 0.8  
Open derivative contracts for natural gas (in MMBtus) 9,500,000   9,500,000   13,200,000
Percentage of natural gas consumption covered by derivatives 84.00%   84.00%    
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position 0.5   0.5   0
Total derivative assets          
Gross and net amounts presented in consolidated balance sheet 0.7   0.7   7.8
Total derivative liabilities          
Gross and net amounts presented in consolidated balance sheet 0.5   0.5    
Gross amounts not offset in consolidated balance sheet          
Financial instruments 0.5   0.5    
Cash collateral received 0   0    
Net amount 0.2   0.2   7.8
Gross amounts not offset in consolidated balance sheet          
Financial instruments 0.5   0.5    
Cash collateral pledged 0   0    
Net amount 0   0    
Net assets (liabilities)          
Gross and net amounts presented in consolidated balance sheet 0.2   0.2   7.8
Natural gas derivatives not designated as hedging instruments
         
Derivative financial instruments          
Unrealized gains in other current assets 0.7   0.7   7.8
Unrealized losses in other current liabilities (0.5)   (0.5)    
Net unrealized derivative gains 0.2   0.2   7.8
Unrealized mark-to-market (losses) gains (4.9) (2.9) (8.9) 0.8  
Realized gains (losses) 4.9 2.7 14.8 1.1  
Net derivative gains (losses)   $ (0.2) $ 5.9 $ 1.9