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Decommissioning Liability (Tables)
 12 Months Ended
Dec. 31, 2017
Other Provisions, Contingent Liabilities And Contingent Assets [Abstract]  
Decommissioning Liability
The following table reconciles the decommissioning liability:
($ millions)
2017

 
  2016

 
Decommissioning liability, beginning of year
1,314.4

 
1,255.4

 
Liabilities incurred
39.9

 
43.1

 
Liabilities acquired through capital acquisitions
25.1

 
23.7

 
Liabilities disposed through capital dispositions
(66.5
)
 
(10.7
)
 
Liabilities settled
(25.1
)
 
(16.0
)
 
Revaluation of acquired decommissioning liabilities (1)
42.8

 
36.1

 
Change in estimated future costs
2.8

 
(27.1
)
 
Change in discount rate
(7.2
)
 
(14.5
)
 
Accretion expense
30.3

 
25.9

 
Reclassified as liabilities associated with assets held for sale
(4.6
)
 
—

 
Foreign exchange
(7.7
)
 
(1.5
)
 
Decommissioning liability, end of year
1,344.2

 
1,314.4

 
Expected to be incurred within one year
33.7

 
23.7

 
Expected to be incurred beyond one year
1,310.5

 
1,290.7

 
(1)
These amounts relate to the revaluation of acquired decommissioning liabilities at the end of the period using a risk-free discount rate. At the date of acquisition, acquired decommissioning liabilities are fair valued.