Quarterly report pursuant to Section 13 or 15(d)

Sale of Weald Basin Assets (Details)

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Sale of Weald Basin Assets (Details)
£ in Thousands, shares in Millions, $ in Millions
6 Months Ended 12 Months Ended
Aug. 11, 2016
GBP (£)
shares
Jun. 10, 2016
GBP (£)
Dec. 31, 2016
USD ($)
Jun. 30, 2016
GBP (£)
Celtique Claim        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Settlement costs £ 500     £ 500
Subsidiaries | Celtique Claim        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Settlement costs 500      
Subsidiaries | Weald ATA, IoW ATA, and Settlement Agreement        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Cash proceeds £ 446      
Subsidiaries | UK Oil and Gas Investments | UK Oil and Gas Investments PLC | Weald ATA, IoW ATA, and Settlement Agreement        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Shares issued (in shares) | shares 50.9      
Investment, face amount £ 713     £ 992
Gain on disposal of discontinued operations before tax | $     $ 1.1  
Asset Transfer Agreement | Subsidiaries | UK Oil and Gas Investments PLC | UK Oil and Gas Investments        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Ownership percentage, sold in period   50.00%    
Total consideration   £ 1,800    
Asset Transfer Agreement | Subsidiaries | UK Oil and Gas Investments PLC | Offshore Petroleum Licence        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Ownership percentage, sold in period   22.50%    
Total consideration   £ 16