A liability for the present value of an ARO is recognized when incurred if the liability can be reasonably estimated. Uncertainty about the timing or method of settlement of a conditional ARO is factored into the measurement of the liability when sufficient information exists, but such uncertainty is not a basis upon which to avoid liability recognition.

The legal obligations associated with the retirement of long-lived tangible assets that result from their acquisition, construction, development and normal operation relate primarily to DESC’s regulated utility operations. As of December 31, 2019, DESC has recorded AROs of $177 million for nuclear plant decommissioning. At December 31, 2019, DESC had $214 million in a trust for its two-thirds share of decommissioning activities. In addition, DESC has recorded AROs of $312 million for other conditional obligations primarily related to other generation, transmission and distribution properties, including gas pipelines. All of the amounts

recorded are based upon estimates which are subject to varying degrees of precision, particularly since such payments will be made many years in the future.

A reconciliation of the beginning and ending aggregate carrying amount of AROs is as follows:

(millions)	2019			2018
Beginning balance	$	541		$	529
Liabilities settled		(29	)		(15	)
Accretion expense		23			23
Revisions in estimated cash flows⁽¹⁾		(46	)		4
Ending balance	$	489		$	541

(1) The decrease in 2019 reflects a change in the estimated timing of cash flows for interim pipeline replacements and DOE recoveries.