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Reconciliation of Asset Retirement Obligation Liability (Tables)
 12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation [Abstract]  
Schedule of change in asset retirement obligation liability [Table Text Block]
A reconciliation of the Company’s total asset retirement obligation liability is as follows:
 
As of December 31,
 
2018
 
2017
 
(in thousands)
Beginning asset retirement obligations
$
114,470

 
$
123,307

Liabilities incurred (1)
4,054

 
7,588

Liabilities settled (2)
(33,024
)
 
(30,432
)
Accretion expense
4,438

 
5,988

Revision to estimated cash flows
4,256

 
8,019

Ending asset retirement obligations (3)(4)
$
94,194

 
$
114,470

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(1) 
Reflects liabilities incurred through drilling activities and acquisitions of drilled wells.
(2) 
Reflects liabilities settled through plugging and abandonment activities and divestitures of properties.
(3) 
Balance as of December 31, 2017, included $11.4 million of asset retirement obligations associated with oil and gas properties held for sale.
(4) 
Balances as of December 31, 2018, and 2017, included $2.3 million and $75,000, respectively, related to the Company’s current asset retirement obligation liability, which is recorded in the accounts payable and accrued expenses line item on the accompanying balance sheets.