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Asset Retirement Obligations (ARO) (Tables)
9 Months Ended
Sep. 30, 2023
Asset Retirement Obligation Disclosure [Abstract]  
Schedule of 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.
 
(Millions of dollars)September 30,
2023
December 31,
2022
Balance at beginning of period$43.3 $39.2 
Accretion expense2.2 2.7 
Settlements of liabilities(3.1)(2.3)
Liabilities incurred1.6 3.7 
Balance at end of period$44.0 $43.3