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Income taxes - Factors affecting the taxation charge for the year (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Abstract]      
Standard corporate tax rate 19.25% 20.00% 20.25%
Profit on operations before taxation $ 11.9 $ 27.9 $ 25.6
Profit on operations at 2017 standard rate of corporation tax in the U.K. of 19.25% (2016: 20% and 2015: 20.25%) 2.3 5.6 5.2
(Income not taxable) / non-deductible expenses 0.1 0.2 2.4
Unprovided deferred income taxes 0.3 (2.9) 0.0
Foreign tax rate differences 4.3 2.7 2.6
Effect of U.S. tax reform (6.0) 0.0 0.0
Adjustment in respect of previous years (0.6) 0.4 (0.7)
Tax on profit on operations 0.4 $ 6.0 $ 9.5
Deferred tax credit from US tax reform $ 6.0