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Decommissioning Liability (Tables)
 12 Months Ended
Dec. 31, 2019
Other Provisions, Contingent Liabilities And Contingent Assets [Abstract]  
Decommissioning Liability
($ millions)
2019

 
2018

 
Decommissioning liability, beginning of year
1,230.7

 
1,344.2

 
Liabilities incurred
30.7

 
38.6

 
Liabilities acquired through capital acquisitions
0.7

 
0.4

 
Liabilities disposed through capital dispositions
(223.6
)
 
(68.3
)
 
Liabilities settled
(28.7
)
 
(25.3
)
 
Revaluation of acquired decommissioning liabilities (1)
0.9

 
0.6

 
Change in estimated future costs
6.2

 
(79.9
)
 
Change in discount rate
115.3

 
(20.2
)
 
Accretion expense
23.5

 
30.6

 
Reclassified as liabilities associated with assets held for sale
(7.9
)
 
—

 
Foreign exchange
(3.8
)
 
10.0

 
Decommissioning liability, end of year
1,144.0

 
1,230.7

 
Expected to be incurred within one year
35.0

 
26.9

 
Expected to be incurred beyond one year
1,109.0

 
1,203.8

 
(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.