Asset retirement obligations (AROs) are legal obligations associated with the retirement of long-lived assets resulting from the acquisition, construction, development and/or normal use of the underlying assets, including legal obligations for land reclamation. Recognition of a liability for an ARO is required in the period in which it is incurred at its estimated fair value. The associated asset retirement costs are capitalized as part of the carrying amount of the underlying asset and depreciated over the estimated useful life of the asset. The liability is accreted through charges to operating expenses. If the ARO is settled for other than the carrying amount of the liability, we recognize a gain or loss on settlement.

For the years ended December 31, ARO operating costs related to accretion of the liabilities and depreciation of the assets are as follows:



in millions	2022	2021	2020
ARO Operating Costs
Accretion	$ 14.1	$ 13.1	$ 12.4
Depreciation	9.4	10.6	8.6
Total	$ 23.5	$ 23.7	$ 21.0

ARO operating costs are reported in cost of revenues. AROs are reported within other noncurrent liabilities in our accompanying Consolidated Balance Sheets.

Reconciliations of the carrying amounts of our AROs for the years ended December 31 are as follows:



in millions	2022	2021
Asset Retirement Obligations
Balance at beginning of year	$ 315.2	$ 283.2
Liabilities incurred	8.2	19.2
Liabilities settled	(8.8)	(11.0)
Accretion expense	14.1	13.1
Revisions, net	(17.4)	10.7
Balance at end of year	$ 311.3	$ 315.2