Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2017

Asset Retirement Obligation Disclosure [Abstract]

Schedule of change in asset retirement obligation

During 2017 and 2016, our overall asset retirement obligation changed as follows (in thousands):

	2017			2016
Beginning balance	$	275,452		$	323,026
Accretion (1)	104,659			35,740
New obligations	28,447			7,995
Changes in estimates of existing obligations (2)	(38,470		)	(85,514		)
Property dispositions/obligations settled	(6,132		)	(5,795		)
Ending balance	$	363,956		$	275,452

(1) The increase in accretion for 2017 includes the 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 changes in estimates of existing obligations reflect a decrease of $38 million and $86 million for 2017 and 2016, respectively. The decrease in 2017 is primarily due to a decrease in current estimates for offshore removal costs. The decrease in 2016 is due primarily to revisions in the estimated remaining life of assets and the current estimate to the inflation rate.