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Risk Management (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Derivative [Line Items]          
Collateral posted for commodity derivative instruments $ 5.7   $ 5.7   $ 4.3
Commodity contract          
Derivative [Line Items]          
Gain (loss) on derivative instruments not designated as hedging 24.1 $ 2.1 22.6 $ 4.1  
Collateral posted for commodity derivative instruments 0.1   0.1    
Commodity contract with credit contingent features          
Derivative [Line Items]          
Aggregate fair value of commodity derivative instruments 30.1   30.1   13.9
NYMEX Derivative Liability          
Derivative [Line Items]          
Aggregate fair value of commodity derivative instruments         14.3
Derivative Asset 30.7   30.7    
NYMEX Margin Deposit          
Derivative [Line Items]          
NYMEX margin deposits $ 25.2   $ 25.2   $ 4.2
ERROR in label resolution. | Maximum          
Derivative [Line Items]          
Remaining maturity     35 months    
Percent of contracts expiring in the next twelve months     87.00%