Asset Retirement Obligations (Tables)

12 Months Ended

Dec. 31, 2017

Asset Retirement Obligation Disclosure [Abstract]

Change in asset retirement obligations

The following table shows the change in our asset retirement obligations for 2017 and 2016 (dollars in thousands):

	2017		2016
Asset retirement obligations at the beginning of year	$	624,475	$	443,576
Changes attributable to:
Accretion expense	33,104		26,656
Settlements	—		(15,732		)
Estimated cash flow revisions	21,950		151,046
Newly incurred or acquired obligations	—		18,929
Asset retirement obligations at the end of year	$	679,529	$	624,475