Segment information - Assets, Long Lived Assets, Revenues, Net Assets by Geographic Area (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
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, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment $ 3,059.6       $ 1,994.8       $ 3,059.6 $ 1,994.8 $ 1,946.7
Long-Lived Assets by Geographic Area 640.9       531.0       640.9 531.0 549.0
Revenues by Geographic Area and Service Type 903.2 $ 849.5 $ 805.9 $ 788.4 803.5 $ 755.8 $ 739.5 $ 721.8 3,347.0 3,020.6 3,061.4
Net assets outside the U.S. 1,191.0       684.1       1,191.0 684.1 647.8
Reportable Segments:                      
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment 2,357.3       1,622.6       2,357.3 1,622.6 1,567.4
Revenues by Geographic Area and Service Type                 3,192.9 2,908.4 2,976.9
Corporate items                      
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment 643.6       321.3       643.6 321.3 332.4
North America | Reportable Segments:                      
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment 733.5       629.4       733.5 629.4 638.4
Revenues by Geographic Area and Service Type                 1,254.2 1,210.3 1,224.0
South America | Reportable Segments:                      
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment 740.5       371.4       740.5 371.4 315.0
Revenues by Geographic Area and Service Type                 924.6 718.7 719.3
Rest of World | Reportable Segments:                      
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment 883.3       621.8       883.3 621.8 614.0
Revenues by Geographic Area and Service Type                 1,014.1 979.4 1,033.6
Mexico                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 99.6       78.1       99.6 78.1 93.2
Revenues by Geographic Area and Service Type                 327.2 296.3 333.7
Net assets outside the U.S. 133.7       86.1       133.7 86.1 86.1
France                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 84.1       67.5       84.1 67.5 68.1
Revenues by Geographic Area and Service Type                 429.4 424.5 431.5
Net assets outside the U.S. 219.4       102.1       219.4 102.1 87.1
Brazil                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 57.2       45.3       57.2 45.3 34.0
Revenues by Geographic Area and Service Type                 434.6 344.9 336.4
Net assets outside the U.S. 151.3       106.3       151.3 106.3 87.9
ARGENTINA                      
Segment Reporting Information [Line Items]                      
Revenues by Geographic Area and Service Type                 250.3 156.4 158.4
Net assets outside the U.S. 234.0       28.6       234.0 28.6 28.3
Venezuela                      
Segment Reporting Information [Line Items]                      
Revenues by Geographic Area and Service Type                 154.1 109.4 84.5
Venezuela | Other items not allocated to segments:                      
Segment Reporting Information [Line Items]                      
Assets held by Reportable Segment 58.7       50.9       58.7 50.9 46.9
Revenues by Geographic Area and Service Type                 154.1 109.4 84.5
Canada                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 46.7       42.4       46.7 42.4 38.9
Revenues by Geographic Area and Service Type                 151.2 150.2 153.7
Net assets outside the U.S. 63.3       57.6       63.3 57.6 50.3
Other                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 146.5       127.2       146.5 127.2 122.7
Revenues by Geographic Area and Service Type                 824.4 775.1 826.7
Subtotal                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 434.1       360.5       434.1 360.5 356.9
Revenues by Geographic Area and Service Type                 2,571.2 2,256.8 2,324.9
U.S.                      
Segment Reporting Information [Line Items]                      
Long-Lived Assets by Geographic Area 206.8       170.5       206.8 170.5 192.1
Revenues by Geographic Area and Service Type                 775.8 763.8 736.5
Other Rest of World countries                      
Segment Reporting Information [Line Items]                      
Net assets outside the U.S. 273.1       205.0       273.1 205.0 221.2
Other South American countries                      
Segment Reporting Information [Line Items]                      
Net assets outside the U.S. $ 116.2       $ 98.4       $ 116.2 $ 98.4 $ 86.9