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

The following table provides a summary of changes in asset retirement obligations, which are included in other liabilities in the accompanying consolidated balance sheets. Revisions in estimates for the periods presented relate primarily to revisions of current cost estimates, inflation rates and/or discount rates.

 

 

Years Ended December 31,

 

 

2014

 

  

2013

 

 

2012

 

 

(in thousands)

 

Asset retirement obligations, beginning of period

$

4,521

 

 

$

5,335

 

 

$

3,409

 

Additions

 

1,789

 

 

 

—

 

 

 

1,816

 

Revisions

 

6,705

 

 

 

(1,314

)

 

 

(133

)

Accretion expense

 

2,058

 

 

 

500

 

 

 

243

 

Deletions

 

—

 

 

 

—

 

 

 

—

 

Asset retirement obligations, end of period

$

15,073

 

 

$

4,521

 

 

$

5,335