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