Asset Retirement Obligations

The Company’s asset retirement obligations arise from the Federal Surface Mining Control and Reclamation Act of 1977 and similar state statutes, which require that mine property be restored in accordance with specified standards and an approved reclamation plan. The required reclamation activities to be performed are outlined in the Company’s mining permits. These activities include reclaiming the pit and support acreage at surface mines, sealing portals at underground mines, reclaiming refuse areas and slurry ponds and water treatment.

The following table describes the changes to the Company’s asset retirement obligation liability:



	Year Ended December 31, 2018			Year Ended December 31, 2017
(In thousands)
Balance at beginning of period (including current portion)	$	328,695		$	356,742
Accretion expense	27,970			30,209
Obligations of divested operations	—			(12,569		)
Adjustments to the liability from changes in estimates	(100,728		)	(23,215		)
Liabilities settled	(12,520		)	(22,472		)
Fresh start accounting adjustment	—			—
Balance at period end	$	243,417		$	328,695
Current portion included in accrued expenses	(13,113		)	(19,840		)
Noncurrent liability	$	230,304		$	308,855

The reduction in the asset retirement obligation during the year ended December 31, 2018 primarily relates to a revised mining and reclamation plan at the Company’s Black Thunder Mine. The revised plan provides for accelerated mine reclamation during the ordinary mining process, which significantly reduces the size of the mine’s open pit at the time of final reclamation. The change reduced the asset retirement obligation, and corresponding deferred mine development asset on the Company’s balance sheet by $95.6 million.

As of December 31, 2018, the Company had $536.2 million in surety bonds outstanding and no letters of credit to secure reclamation bonding obligations. Additionally, the Company has posted $0.6 million in cash as collateral related to reclamation surety bonds; this amount is recorded within “Noncurrent assets” on the Consolidated Balance Sheet.