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Asset Retirement Obligations Reconciliation of Asset Retirement Obligation Liability (Tables)
 12 Months Ended
Dec. 31, 2015
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,
 
2015
 
2014
 
(in thousands)
Beginning asset retirement obligation
$
122,124

 
$
121,186

Liabilities incurred
14,471

 
13,506

Liabilities settled
(24,781
)
 
(11,372
)
Accretion expense
5,091

 
6,090

Revision to estimated cash flows
23,969

 
(7,286
)
Ending asset retirement obligation
$
140,874

 
$
122,124