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Asset retirement obligations (Tables)
 12 Months Ended
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations
The following table presents the balance and activity of our asset retirement obligations:
 
Liability for asset retirement obligations as of January 1, 2018
$
35,990

Liabilities incurred in current period
689

Liabilities settled and disposed in current period
(17,868
)
Revisions in estimated cash flows
2,452

Accretion expense
1,884

Liability for asset retirement obligations as of December 31, 2018
$
23,147

Liabilities incurred in current period
448

Liabilities settled and disposed in current period
(2,305
)
Revisions in estimated cash flows
388

Accretion expense
1,478

Liability for asset retirement obligations as of December 31, 2019
$
23,156

Less current portion included in accounts payable and accrued liabilities
2,083

Asset retirement obligations, long-term
$
21,073