Asset Retirement Obligations (Tables)
12 Months Ended
Dec. 31, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Summary of Changes in Asset Retirement Obligations

The following table presents the changes in the asset retirement obligations for the years ended December 31, 2014, 2013, and 2012:

 

 

 

2014

 

 

2013

 

 

2012

 

 

 

(in thousands)

 

Asset retirement obligations at beginning of period

 

$

101,436

 

 

$

93,031

 

 

$

83,696

 

Liabilities added from acquisitions or drilling

 

 

5,815

 

 

 

2,116

 

 

 

5,967

 

Liabilities removed upon sale of wells

 

 

 

 

 

 

 

 

(1,795

)

Liabilities settled

 

 

(651

)

 

 

(20

)

 

 

(91

)

Accretion expense

 

 

5,773

 

 

 

4,988

 

 

 

4,458

 

Revision of estimates

 

 

329

 

 

 

1,321

 

 

 

796

 

Asset retirement obligations at end of period

 

$

112,702

 

 

$

101,436

 

 

$

93,031