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Derivative Losses (Tables)
 12 Months Ended
Dec. 31, 2017
Analysis of income and expense [abstract]  
Schedule of Derivative Instruments
($ millions)
2017

 
2016

 
Realized gains
101.2

 
468.7

 
Unrealized losses
(163.6
)
 
(706.8
)
 
Derivative losses
(62.4
)
 
(238.1
)