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Asset retirement obligations
 9 Months Ended
Sep. 30, 2014
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations
Asset retirement obligations
The following table provides a summary of our asset retirement obligation activity during the nine months ended September 30, 2014 and 2013. 
 
 
Nine months ended September 30,
 
 
2014
 
2013
Beginning balance
 
$
55,179

 
$
49,214

Liabilities incurred in current period
 
4,844

 
1,036

Liabilities settled and disposed in current period
 
(16,807
)
 
(4,969
)
Revisions in estimated cash flows
 
2,274

 
—

Accretion expense
 
2,991

 
2,968

Ending balance
 
48,481

 
48,249

Less current portion included in accounts payable and accrued liabilities
 
4,997

 
2,900

 
 
$
43,484

 
$
45,349


See Note 6—Fair value measurements for additional information regarding fair value assumptions associated with our asset retirement obligations.