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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2013
Asset Retirement Obligation Disclosure [Abstract]  
Changes to the Company's asset retirement obligation liability
The following table describes all changes to the Company’s asset retirement obligation liability (in thousands):
 
Year Ended December 31,
 
2013
 
2012
Asset retirement obligation, beginning of period
$
27,260

 
$
30,427

Liabilities assumed
15,319

 
—

Liabilities incurred
498

 
892

Liabilities settled
(3,335
)
 
(2,627
)
Accretion expense
1,753

 
2,078

Revisions in estimated cash flows
7,041

 
(3,510
)
Asset retirement obligation, end of period
48,536

 
27,260

Less: current portion of asset retirement obligation
(3,113
)
 
(2,351
)
Long-term asset retirement obligation
$
45,423

 
$
24,909