Annual report pursuant to Section 13 and 15(d)

Income taxes - Reconciliation of Effective Tax Rate (Details)

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Income taxes - Reconciliation of Effective Tax Rate (Details) - Her Majesty's Revenue and Customs (HMRC)
12 Months Ended
Apr. 01, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income taxes        
UK tax rate 19.00% 19.00% 19.25% 20.00%
Reimbursable tax credits   3.50% 2.60% 1.60%
Surrender of R&D expenditures for R&D tax credit refund   (10.00%) (8.40%) (5.90%)
Change in valuation allowances   (17.50%) (13.50%) (14.70%)
Difference in tax rates   (1.30%) (1.40%) (2.60%)
R&D tax credits generated   5.10% 0.70%  
Other   0.80% 0.10% 0.30%
Effective income tax rate   (0.50%) (0.60%) (1.30%)