Asset Retirement Obligations

12 Months Ended

Dec. 31, 2019

Asset Retirement Obligation Disclosure [Abstract]

Asset Retirement Obligations

12. ASSET RETIREMENT OBLIGATIONS

The following table provides a summary of changes in asset retirement obligations:

	Year Ended December 31,
thousands	2019			2018
Carrying amount of asset retirement obligations at beginning of year	$	325,962		$	154,571
Liabilities incurred	27,360			34,558
Liabilities settled	(17,104		)	(12,432		)
Accretion expense	13,599			7,909
Revisions in estimated liabilities	9,051			141,356
Carrying amount of asset retirement obligations at end of year	$	358,868		$	325,962

The liabilities incurred for the year ended December 31, 2019, represented additions in asset retirement obligations primarily due to capital expansions at the West Texas and DJ Basin complexes. Revisions in estimated liabilities for the year ended December 31, 2019, primarily related to (i) changes in expected settlement costs at the West Texas and DJ Basin complexes and (ii) changes to the expected abandonment timing of transportation assets in Wyoming.

The liabilities incurred for the year ended December 31, 2018, represented additions in asset retirement obligations primarily due to capital expansions at the West Texas and DJ Basin complexes, the DBM water systems, and the DBM oil system. Revisions in estimated liabilities for the year ended December 31, 2018, primarily included (i) $71.8 million related to changes in expected settlement costs and timing, primarily at the DJ Basin and West Texas complexes and the MGR assets, and (ii) $43.4 million related to the shutdown of the Third Creek gathering system during the second quarter of 2018. See Note 1 for further information.