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)
6 Months Ended 12 Months Ended
Apr. 01, 2017
Dec. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Jun. 30, 2015
Income taxes          
UK tax rate 19.00% 20.00% 19.25% 20.00% 20.75%
Permanent differences relating to reimbursable tax credits   1.60% 2.60% 1.60% 2.30%
Permanent differences relating to foreign exchange   (8.70%)     3.40%
Surrender of R&D expenditures for R&D tax credit refund   (4.50%) (8.40%) (5.90%) (5.50%)
Change in valuation allowances   (10.80%) (13.50%) (14.70%) (20.70%)
Other   2.60% (0.60%) (2.30%) (1.40%)
Effective income tax rate   0.20% (0.60%) (1.30%) (1.10%)