Annual report pursuant to Section 13 and 15(d)

Income taxes - Change in Tax Rate (Details)

v3.8.0.1
Income taxes - Change in Tax Rate (Details)
$ in Thousands, £ in Millions
6 Months Ended 12 Months Ended
Apr. 01, 2020
Dec. 31, 2017
USD ($)
Apr. 01, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2017
GBP (£)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
Jun. 30, 2015
Income taxes                  
Effect of changes in tax rates on Consolidated statement of Operations (in dollars)             $ 0    
Percentage on taxable profit   50.00%         50.00%    
Operating Loss Carryforwards And Tax Credit Carryforward Annual Allowance | £           £ 5      
Her Majesty's Revenue and Customs (HMRC)                  
Income taxes                  
Tax rate (as a percent)     19.00%   20.00% 19.25% 19.25% 20.00% 20.75%
Tax rate, deferred taxes (as a percent)   17.00%              
Her Majesty's Revenue and Customs (HMRC) | Forecast                  
Income taxes                  
Tax rate (as a percent) 17.00%                
Internal Revenue Service (IRS)                  
Income taxes                  
Tax rate (as a percent)         34.00% 34.00% 34.00% 34.00% 34.00%
Tax rate, deferred taxes (as a percent)   21.00%   34.00%          
Decrease in net deferred tax asset and corresponding valuation allowance   $ 1,800         $ 1,800    
Tax credit carried forward period           20 years 20 years