Asset Retirement Obligations (Tables)

6 Months Ended

Jun. 30, 2016

Asset Retirement Obligation Disclosure [Abstract]

Change in asset retirement obligations

The following schedule shows the change in our asset retirement obligations for the six months ended June 30, 2016 (dollars in thousands): 

Asset retirement obligations at January 1, 2016	$	443,576
Changes attributable to:
Accretion expense	13,112
Settlements	(5,224		)
Newly incurred liabilities	10,373
Asset retirement obligations at June 30, 2016	$	461,837