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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2017
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
 
2017
 
2016
Carrying amount of asset retirement obligations at beginning of year
 
$
142,407

 
$
130,631

Liabilities incurred
 
16,153

 
5,515

Liabilities settled
 
(10,468
)
 
(10,650
)
Accretion expense
 
6,956

 
6,794

Revisions in estimated liabilities
 
(9,350
)
 
10,117

Carrying amount of asset retirement obligations at end of year
 
$
145,698

 
$
142,407