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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2016
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
 
2016
 
2015
Carrying amount of asset retirement obligations at beginning of year
 
$
130,631

 
$
119,855

Liabilities incurred
 
5,515

 
9,490

Liabilities settled
 
(10,650
)
 
(7,905
)
Accretion expense
 
6,794

 
6,381

Revisions in estimated liabilities
 
10,117

 
2,810

Carrying amount of asset retirement obligations at end of year
 
$
142,407

 
$
130,631