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Commodity Derivative Gains (Losses) (Tables)
 12 Months Ended
Dec. 31, 2019
Analysis of income and expense [abstract]  
Schedule of Derivative Instruments
($ millions)
2019

 
2018

 
Realized gains (losses)
43.4

 
(259.8
)
 
Unrealized gains (losses)
(104.4
)
 
203.4

 
Commodity derivative losses
(61.0
)
 
(56.4
)