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Other Income (Loss) (Tables)
 12 Months Ended
Dec. 31, 2017
Analysis of income and expense [abstract]  
Schedule of Other Income (Loss)
($ millions)
2017

 
2016

 
Unrealized gain (loss) on long-term investments
(3.4)

 
5.5

 
Gain (loss) on capital dispositions
31.1

 
(15.3
)
 
Other gain
0.1

 
3.2

 
Other income (loss)
27.8

 
(6.6
)