ASSET RETIREMENT OBLIGATION

We recognize an estimated liability for future costs associated with the abandonment and reclamation of our mining properties. A liability for the fair value of an asset retirement obligation and a corresponding increase to the carrying value of the related long-lived asset are recorded as the mining operations occur or the assets are acquired.

Our asset retirement obligation is based on the estimated cost to abandon and reclaim the mining operations, the economic life of the properties, and federal and state regulatory requirements. The liability is discounted using credit adjusted risk-free rate estimates at the time the liability is incurred or when there are revisions to estimated costs. The credit adjusted risk-free rates used to discount our abandonment liabilities range from 6.9% to 9.7%. Revisions to the liability occur due to construction of new or expanded facilities, changes in estimated abandonment costs or economic lives, or if federal or state regulators enact new requirements regarding the abandonment or reclamation of mines.

Following is a table of the changes to our asset retirement obligation for the following periods (in thousands):



	Three Months Ended September 30,				Nine Months Ended September 30,
	2017		2016		2017		2016
Asset retirement obligation, at beginning of period	$	20,754	$	23,832	$	19,976	$	22,951
Liabilities settled	—		—		—		(3		)
Accretion of discount	390		442		1,168		1,326
Total asset retirement obligation, at end of period	$	21,144	$	24,274	$	21,144	$	24,274

The undiscounted amount of asset retirement obligation was $59.3 million as of September 30, 2017.