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Asset Retirement Obligations (ARO) (Tables)
 12 Months Ended
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligation
A reconciliation of the beginning and ending aggregate carrying amount of the ARO is shown in the following table.
 
 
December 31,
(Millions of dollars)
 
2019
 
2018
Balance at beginning of period
 
$
30.7

 
$
28.2

Accretion expense
 
2.1

 
2.0

Settlement of liabilities
 
(0.4
)
 
(0.3
)
Liabilities incurred
 
0.4

 
0.8

Balance at end of period
 
$
32.8

 
$
30.7