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Asset retirement obligations
 3 Months Ended
Mar. 31, 2020
Asset Retirement Obligation Disclosure [Abstract]  
Asset retirement obligations Asset retirement obligations
See Note 2.l in the 2019 Annual Report for discussion of the Company's significant accounting policies for asset retirement obligations.
The following table reconciles the Company's asset retirement obligation liability associated with tangible long-lived assets for the periods presented:
 
 
Three months ended March 31,
(in thousands)
 
2020
 
2019
Liability at beginning of period
 
$
62,718

 
$
56,882

Liabilities added due to acquisitions, drilling, midstream service asset construction and other
 
886

 
271

Accretion expense
 
1,106

 
1,052

Liabilities settled due to plugging and abandonment
 
(497
)
 
(447
)
Liability at end of period
 
$
64,213

 
$
57,758