Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2018

Asset Retirement Obligation Disclosure [Abstract]

Schedule of change in asset retirement obligation

During 2018 and 2017, our overall asset retirement obligation changed as follows (in thousands):

	2018			2017
Beginning balance	$	363,956		$	275,452
Accretion (1)	32,924			104,659
New obligations	14,162			28,447
Changes in estimates of existing obligations (2)	(8,054		)	(38,470		)
Property dispositions/obligations settled	(8,665		)	(6,132		)
Ending balance	$	394,323		$	363,956

(1) The decrease in accretion for 2018 is due to the 2017 cumulative effect of accretion adjustment associated with new AROs identified in our historical land agreements of $87 million that are not a component of new obligations.

(2) Changes in estimates of existing obligations are primarily due to the annual review process, which considers various factors including inflation rate, current estimates for removal cost, discount rates, and the estimated remaining life of assets. The decrease in 2018 is primarily due to a decrease in current estimates for onshore removal costs. The decrease in 2017 is primarily due to a decrease in current estimates for offshore removal costs.