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Foreign Exchange Gain (Tables)
 12 Months Ended
Dec. 31, 2017
Effect Of Changes In Foreign Exchange Rates [Abstract]  
Schedule of Foreign Exchange Gain (Loss)
($ millions)
2017

 
2016

 
Realized gain (loss)
 
 
 
 
CCS - US dollar long-term debt maturities and interest payments
(39.3
)
 
57.7

 
US dollar long-term debt maturities
54.6

 
(52.4
)
 
Other
(0.6
)
 
(2.5
)
 
Unrealized gain (loss)
 
 
 
 
Translation of US dollar long-term debt
201.2

 
128.0

 
Other
(0.2
)
 
0.5

 
Foreign exchange gain
215.7

 
131.3