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Asset Retirement Obligations (table)
 12 Months Ended
Dec. 31, 2013
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
 
2013
 
2012
Carrying amount of asset retirement obligations at beginning of year
 
$
66,723

 
$
64,345

Liabilities incurred
 
14,143

 
9,414

Liabilities settled
 
(1,943
)
 
(786
)
Accretion expense
 
4,326

 
4,270

Revisions in estimated liabilities
 
(5,214
)
 
(10,520
)
Carrying amount of asset retirement obligations at end of year
 
$
78,035

 
$
66,723