Asset Retirement Obligations (ARO) (Tables)

6 Months Ended

Jun. 30, 2020

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.

(Millions of dollars)	June 30, 2020		December 31, 2019
Balance at beginning of period	$	32.8	$	30.7
Accretion expense	1.1		2.1
Settlements of liabilities	(0.3)		(0.4)
Liabilities incurred	0.1		0.4
Balance at end of period	$	33.7	$	32.8