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Asset Retirement Obligations (Tables)
12 Months Ended
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]  
Schedule Of Changes In Asset Retirement Obligation
 
2018
 
2017
 
2016
Asset retirement obligation at beginning of year
$
17,482

 
$
15,515

 
$
15,997

Liabilities incurred

 
1,171

 
430

Liabilities settled
(66
)
 

 
(1,529
)
Accretion expense
850

 
796

 
617

Asset retirement obligation at end of year
$
18,266

 
$
17,482

 
$
15,515