| Income taxes - Tax Rates (Details) | 12 Months Ended | |||
|---|---|---|---|---|
| Apr. 01, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | |
| Her Majesty's Revenue and Customs (HMRC) | ||||
| Income taxes | ||||
| Tax rate (as a percent) | 19.00% | 19.00% | 19.25% | 20.00% | 
| Internal Revenue Service (IRS) | ||||
| Income taxes | ||||
| Tax rate (as a percent) | 21.00% | 34.00% | 34.00% | |
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- References No definition available. 
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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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| X | ||||||||||
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- Details 
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- Details 
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