Asset Retirement Obligations

12 Months Ended

Dec. 31, 2017

Asset Retirement Obligation Disclosure [Abstract]

Asset Retirement Obligations

11.  ASSET RETIREMENT OBLIGATIONS

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

	Year Ended December 31,
thousands	2017			2016
Carrying amount of asset retirement obligations at beginning of year	$	142,407		$	130,631
Liabilities incurred	16,153			5,515
Liabilities settled	(10,468		)	(10,650		)
Accretion expense	6,956			6,794
Revisions in estimated liabilities	(9,350		)	10,117
Carrying amount of asset retirement obligations at end of year	$	145,698		$	142,407

The liabilities incurred for the year ended December 31, 2017, represented additions in asset retirement obligations primarily due to (i) capital expansions at the DJ Basin and DBM complexes and the DBJV system, (ii) the Property Exchange in March 2017 and (iii) the start-up of the DBM water systems in 2017. Revisions in estimated liabilities for the year ended December 31, 2017, were related to (i) changes in expected settlement costs and timing primarily at the Hilight system and the DJ Basin and DBM complexes, and (ii) changes in property lives primarily at the Granger, DJ Basin and DBM complexes and the Hilight and DBJV systems.

The liabilities incurred for the year ended December 31, 2016, represented additions in asset retirement obligations primarily due to capital expansions at the DJ Basin and DBM complexes and the DBJV system. Revisions in estimated liabilities for the year ended December 31, 2016, were related to (i) changes in expected settlement costs and timing primarily at the MGR assets, Granger complex and the Hilight and Springfield systems, and (ii) changes in property lives primarily at the DJ Basin and DBM complexes and the Hilight, Springfield and Haley systems.