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Risk Management (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Derivative [Line Items]      
Collateral posted for commodity derivative instruments $ 7.1   $ 16.8
Price Risk Contracts | Maximum      
Derivative [Line Items]      
Remaining maturity 36 months    
Percent of contracts expiring in the next twelve months 81.00%    
Commodity contract      
Derivative [Line Items]      
Gain (loss) on derivative instruments not designated as hedging $ 1.2 $ 2.9  
Collateral posted for commodity derivative instruments 0.3    
Commodity contract with credit contingent features      
Derivative [Line Items]      
Aggregate fair value of commodity derivative instruments 3.8   3.3
NYMEX Derivative Liability      
Derivative [Line Items]      
Aggregate fair value of commodity derivative instruments 3.1   20.8
NYMEX Margin Deposit      
Derivative [Line Items]      
NYMEX margin deposits $ 9.5   $ 26.7