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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2018
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
 
2018
 
2017
Carrying amount of asset retirement obligations at beginning of year
 
$
154,571

 
$
150,082

Liabilities incurred
 
34,558

 
17,932

Liabilities settled
 
(12,432
)
 
(10,468
)
Accretion expense
 
7,909

 
7,339

Revisions in estimated liabilities
 
141,356

 
(10,314
)
Carrying amount of asset retirement obligations at end of year
 
$
325,962

 
$
154,571