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Summary of Significant Accounting Policies (Tables)
 12 Months Ended
Dec. 31, 2017
Accounting Policies [Abstract]  
Net Changes in Capitalized Exploratory Well Costs
Net changes in capitalized exploratory well costs are reflected in the following table for the periods presented:
 
December 31,
 
2017
 
2016
 
2015
 
(In thousands)
Beginning of period
$
2,097

 
$
2,097

 
$
34,522

Exploratory well cost additions (pending determination of proved reserves)
10

 
—

 
51,803

Exploratory well cost reclassifications (successful determination of proved reserves)
(571
)
 
—

 
(84,228
)
Exploratory well dry hole costs (unsuccessful in adding proved reserves)
(1,536
)
 
—

 
—

End of period
$
—

 
$
2,097

 
$
2,097