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Income Tax Credit - Schedule of Reconciliation of Accounting Loss per Statement of Operations (Parenthetical) (Details)
12 Months Ended
Apr. 01, 2017
Oct. 26, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
U.K.          
Reconciliation Of Accounting Profit Multiplied By Applicable Tax Rates [Line Items]          
Applicable tax rate 19.00% 20.00% 19.25% 20.00% 20.25%