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Dividends paid and proposed (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of non-adjusting events after reporting period [line items]                                
Dividends declared and paid during the year   $ 3.4 $ 3.3 $ 3.3 $ 3.3 $ 3.3 $ 3.3 $ 3.3 $ 3.4 $ 2.7 $ 2.7 $ 2.7 $ 2.7 $ 13.3 $ 13.3 $ 10.8
Dividends declared and paid during the year (USD per share)   $ 0.125 $ 0.125 $ 0.125 $ 0.125 $ 0.125 $ 0.125 $ 0.125 $ 0.125 $ 0.1 $ 0.1 $ 0.1 $ 0.1      
Dividends                                
Disclosure of non-adjusting events after reporting period [line items]                                
Dividends declared and paid during the year         $ 3.4                      
Dividends declared and paid during the year (USD per share) $ 0.125