XML 110 R91.htm IDEA: XBRL DOCUMENT v3.8.0.1
Income taxes - Factors that may affect future taxation charge (Details) - USD ($)
$ in Millions
 12 Months Ended
Apr. 01, 2020
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Unused tax losses for which no deferred tax asset recognised   $ 81.9 $ 72.1 $ 82.9
Standard corporate tax rate   19.25% 20.00% 20.25%
Certain trading and capital losses and other temporary timing difference        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Unused tax losses for which no deferred tax asset recognised   $ 13.3 $ 12.3 $ 14.2
U.K.        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Unused tax losses for which no deferred tax asset recognised   43.6 35.3 52.9
Non-U.K.        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Unused tax losses for which no deferred tax asset recognised   $ 38.3 $ 36.8 $ 30.0
Changes in tax rates or tax laws enacted or announced        
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]        
Standard corporate tax rate 17.00%