Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2016

Asset Retirement Obligation Disclosure [Abstract]

Change in asset retirement obligations

The following table shows the change in our asset retirement obligations for 2016 and 2015 (dollars in thousands):

	2016			2015
Asset retirement obligations at the beginning of year	$	443,576		$	390,750
Changes attributable to:
Accretion expense	26,656			25,163
Settlements	(15,732		)	(32,048		)
Estimated cash flow revisions	151,046			17,556
Newly incurred or acquired obligations	18,929			42,155
Asset retirement obligations at the end of year	$	624,475		$	443,576