Income tax - Reconciliation between U.K. statutory income tax rate and the effective tax rate (Details)	11 Months Ended	12 Months Ended
Dec. 31, 2019	Jan. 31, 2019	Jan. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
U.K. corporation tax rate	19.00%	19.00%	19.17%
Change in valuation allowance	(11.03%)	11.93%	(3.79%)
Non-deductible expenses	(1.49%)	25.93%	(1.60%)
Refundable R&D tax credit	3.07%	(5.20%)	3.55%
R&D deduction surrendered	(9.52%)	0.00%	(5.27%)
R&D deduction benefit	0.00%	(18.62%)	0.00%
Overseas profits taxed at different rates	(0.16%)	4.25%	(1.26%)
Release of temporary difference relating to intangible assets	0.51%	(10.23%)	0.00%
Reported effective income tax rate	0.38%	(8.97%)	(1.82%)
ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment on adoption	0.00%	(36.13%)	(12.65%)
ASC 842
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adjustment on adoption	0.00%	0.10%	0.03%

Effective Income Tax Rate Reconciliation, Effect Of Adoption Of Accounting Principle, Percent

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Effective Income Tax Rate Reconciliation, Release Of Temporary Differences For Intangible Assets, Percent

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Effective Income Tax Rate Reconciliation, Tax Credit Refundable, Research, Percent

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Effective Income Tax Rate Reconciliation, Tax Credit Surrendered, Research, Percent

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Publisher FASB
-Name Accounting Standards Codification
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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-SubTopic 10
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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

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