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Asset Retirement Obligations
12 Months Ended
Dec. 31, 2023
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Asset Retirement Obligations
Reconciliations of the Company’s asset retirement obligations are as follows:
 December 31,
 20232022
 (Dollars in millions)
Balance at beginning of period$750.0 $719.8 
Liabilities settled(60.4)(51.7)
Accretion expense61.3 56.2 
Revisions to estimates(48.1)25.7 
Balance at end of period$702.8 $750.0 
Less: Current portion (included in “Accounts payable and accrued expenses”)54.2 84.2 
Noncurrent obligation (included in “Asset retirement obligations, less current portion”)$648.6 $665.8 
Balance at end of period — active locations$541.3 $557.9 
Balance at end of period — closed or inactive locations$161.5 $192.1 
The Company’s reclamation obligations are secured by surety bonds, which are supported by standby letters of credit and restricted cash, and various other forms of collateral. See Note 20. “Financial Instruments, Guarantees With Off-Balance-Sheet Risk and Other Guarantees” for a discussion of the collateral securing the asset retirement obligations.