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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]  
Asset Retirement Obligations Table
The following table provides a summary of changes in asset retirement obligations:
 
 
Year Ended December 31,
thousands
 
2015
 
2014
Carrying amount of asset retirement obligations at beginning of year
 
$
119,855

 
$
82,563

Liabilities incurred
 
9,490

 
14,469

Liabilities settled
 
(7,905
)
 
(4,941
)
Accretion expense
 
6,381

 
5,122

Revisions in estimated liabilities
 
2,810

 
22,642

Carrying amount of asset retirement obligations at end of year
 
$
130,631

 
$
119,855