Annual report pursuant to Section 13 and 15(d)

Income taxes - Net Operating Loss and Expenditure Credit Carryforwards (Details)

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Income taxes - Net Operating Loss and Expenditure Credit Carryforwards (Details) - USD ($)
$ in Thousands
6 Months Ended 12 Months Ended
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2015
Net Operating Loss and Expenditure Credit Carryforwards        
Net operating loss $ (26,410) $ (80,661) $ (72,799) $ (24,641)
Internal Revenue Service (IRS)        
Net Operating Loss and Expenditure Credit Carryforwards        
Tax credit carryforwards   200    
Her Majesty's Revenue and Customs (HMRC)        
Net Operating Loss and Expenditure Credit Carryforwards        
Net operating loss   (135,600)    
Expenditure credit carryforward   $ 600