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Asset Retirement Obligations Reconciliation of Asset Retirement Obligation Liability (Tables)
 12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation [Abstract]  
Schedule of Change in Asset Retirement Obligation [Table Text Block]
A reconciliation of the Company’s total asset retirement obligation liability is as follows:
 
As of December 31,
 
2014
 
2013
 
(in thousands)
Beginning asset retirement obligation
$
121,186

 
$
120,518

Liabilities incurred
13,506

 
18,682

Liabilities settled
(11,372
)
 
(33,129
)
Accretion expense
6,090

 
5,997

Revision to estimated cash flows
(7,286
)
 
9,118

Ending asset retirement obligation
$
122,124

 
$
121,186