Asset Retirement Obligations (Tables)

9 Months Ended

Sep. 30, 2017

Asset Retirement Obligation Disclosure [Abstract]

Change in asset retirement obligations

The following schedule shows the change in our asset retirement obligations for the nine months ended September 30, 2017 (dollars in thousands): 

Asset retirement obligations at January 1, 2017	$	624,475
Changes attributable to:
Accretion expense	24,170
Estimated cash flow revisions	22,211
Asset retirement obligations at September 30, 2017	$	670,856