Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2016

Asset Retirement Obligation Disclosure [Abstract]

Schedule of change in asset retirement obligation

During 2016 and 2015, our overall asset retirement obligation changed as follows (in thousands):

	2016			2015
Beginning balance	$	82,454		$	94,678
Accretion	7,470			6,870
New obligations	150			2,721
Changes in estimates of existing obligations (1)	(29,312		)	(21,815		)
Ending balance	$	60,762		$	82,454

(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 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. The decrease in 2015 is primarily attributed to decreases in current estimates for removal costs and inflation rate and an increase in the discount rate.