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Asset Retirement Obligations Reconciliation of Asset Retirement Obligation Liability (Tables)
 12 Months Ended
Dec. 31, 2013
Asset Retirement Obligation [Abstract]  
Schedule of Change in Asset Retirement Obligation [Table Text Block]
A reconciliation of the Company’s asset retirement obligation liability is as follows:
 
As of December 31,
 
2013
 
2012
 
(in thousands)
Beginning asset retirement obligation
$
120,518

 
$
95,906

Liabilities incurred
18,682

 
13,050

Liabilities settled
(33,129
)
 
(8,101
)
Accretion expense
5,997

 
4,679

Revision to estimated cash flows
9,118

 
14,984

Ending asset retirement obligation
$
121,186

 
$
120,518