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Derivative Financial Instruments (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2013
MMBTU
Dec. 31, 2012
MMBTU
Dec. 31, 2011
Derivative financial instruments      
Unrealized mark-to-market gains (losses) $ 8.6 $ 11.2 $ (11.9)
Open derivative contracts for natural gas (in MMBtus) 13,200,000 9,900,000  
Percentage of natural gas consumption covered by derivatives 91.00%    
Aggregate fair value of the derivative instruments with credit-risk-related contingent features in net liability position 0 0.8  
Total derivative assets      
Gross and net amounts presented in consolidated balance sheet 7.8 0.2  
Gross amounts not offset in consolidated balance sheet      
Financial instruments   0.2  
Net amount 7.8    
Total derivative liabilities      
Gross and net amounts presented in consolidated balance sheet   1.0  
Gross amounts not offset in consolidated balance sheet      
Financial instruments   0.2  
Cash collateral received (pledged) 0 0  
Net amount 0 0.8  
Net assets (liabilities)      
Gross and net amounts presented in consolidated balance sheet 7.8 (0.8)  
Natural gas derivatives not designated as hedging instruments
     
Derivative financial instruments      
Unrealized gains in other current assets 7.8 0.2  
Unrealized losses in other current liabilities   (1.0)  
Net unrealized derivative gains (losses) 7.8 (0.8)  
Unrealized mark-to-market gains (losses) 9.1 10.6 (11.9)
Realized losses (0.8) (24.3) (11.2)
Net derivative gains (losses) $ 8.3 $ (13.7) $ (23.1)