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Asset Retirement Obligation (Tables)
 12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Changes to the Company's asset retirement obligation liability
The following table describes the changes to the Company’s asset retirement obligation (in thousands):
 
Year Ended December 31,
 
2014
 
2013
Asset retirement obligation, beginning of period
$
48,536

 
$
27,260

Liabilities assumed
—

 
15,319

Liabilities incurred
756

 
498

Liabilities settled
(3,623
)
 
(3,335
)
Accretion expense
2,958

 
1,753

Revisions in estimated cash flows
6,343

 
7,041

Asset retirement obligation, end of period
54,970

 
48,536

Less: current portion of asset retirement obligation
(2,756
)
 
(3,113
)
Long-term asset retirement obligation
$
52,214

 
$
45,423