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	$	60,762		$	82,454
Accretion	3,968			7,470
New obligations	149			150
Changes in estimates of existing obligations (1)	2,510			(29,312		)
Property dispositions/obligations settled	(289		)	—
Ending balance	$	67,100		$	60,762

(1) Changes in estimates of existing obligations are primarily due to the annual review process, which considers various factors including inflation rates, current estimates for removal cost, the estimated remaining life of assets, and discount rates. The increase in 2017 is primarily attributed to an increase in the inflation rate and a decrease in the estimated useful life of the assets. The decrease in 2016 is primarily attributed to decreases in current estimates for removal costs and inflation rate and an increase in the estimated life of the assets.