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Acquisition (Details) - Alaska Segment [Member] - Operating Segments [Member] - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2018
Jul. 31, 2018
Interest In WNS Assets and Alpine Pipeline [Member]    
Business Acquisition [Line Items]    
Costs Incurred, Acquisition of Oil and Gas Properties $ 386  
Percent Acquired 22.00%  
Interest In Greater Kuparuk Area [Member] | Scenario Plan [Member]    
Business Acquisition [Line Items]    
Percent Acquired   39.20%
Interest In Kuparuk Transportation Co [Member] | Scenario Plan [Member]    
Business Acquisition [Line Items]    
Percent Acquired   38.00%