Asset retirement obligations associated with the retirement and decommissioning of tangible long-lived assets are required to be recognized as a liability in the period in which a legal obligation is incurred and becomes determinable, with an offsetting increase in the carrying amount of the associated asset.  The cost of the tangible asset, including the initially recognized ARO, is depleted such that the cost of the ARO is recognized over the useful life of the asset.  The fair value of the ARO is measured using expected cash outflows associated with the ARO, discounted at our credit-adjusted risk-free rate when the liability is initially recorded.  Accretion expense is recognized over time as the discounted liability is accreted to its expected settlement value.

 

The following table is a reconciliation of our ARO (in thousands):

 

		Year Ended December 31,
		2019				2018
Asset retirement obligations, beginning of period		$	310,137			$	300,446
Liabilities settled			(11,443	)			(28,617	)
Accretion of discount			19,460				18,431
Liabilities incurred and assumed through acquisition			29,887				4,286
Revisions of estimated liabilities (1) (2)			7,553				15,591
Asset retirement obligations, end of period			355,594				310,137
Less current portion			21,991				24,994
Long-term		$	333,603			$	285,143