Segment information - Summary of Operations by Segment (Details) - USD ($) $ in Millions	3 Months Ended									12 Months Ended
	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, 2016	Dec. 31, 2015		Dec. 31, 2014
Segment Reporting Information [Line Items]
Revenue	$ 721.7	$ 683.3	$ 748.9	$ 666.8	$ 789.9		$ 739.2	$ 764.8	$ 721.2	$ 2,820.7	$ 3,015.1		$ 3,356.4
Operating income (loss)										47.6	(55.1)		154.9
Depreciation and amortization										155.6	180.1		168.6
Total assets	2,021.6				2,130.0	[1]				2,021.6	2,130.0	[1]	2,321.0
Capital expenditures										147.1	213.7		212.8
Intersegment
Segment Reporting Information [Line Items]
Revenue										(22.6)	(49.0)		(58.4)
Operating Segments
Segment Reporting Information [Line Items]
Operating income (loss)										208.4	174.9		233.6
Total assets	1,339.0				1,486.0					1,339.0	1,486.0		1,533.8
Corporate
Segment Reporting Information [Line Items]
Revenue										(22.6)	(49.0)		(58.4)
Operating income (loss)										(160.8)	(230.0)		(78.7)
Total assets	682.6				644.0					682.6	644.0		787.2
Capital expenditures										3.0	1.9		4.9
Services
Segment Reporting Information [Line Items]
Revenue										2,406.3	2,605.6		2,785.7
Operating income (loss)										46.9	61.2		96.0
Depreciation and amortization										81.8	104.8		103.2
Total assets	963.3				1,081.7					963.3	1,081.7		1,099.2
Capital expenditures										74.8	143.3		133.8
Services | Intersegment
Segment Reporting Information [Line Items]
Revenue										0.0	0.1		0.3
Services | Operating Segments
Segment Reporting Information [Line Items]
Revenue										2,406.3	2,605.7		2,786.0
Technology
Segment Reporting Information [Line Items]
Revenue										414.4	409.5		570.7
Operating income (loss)										161.5	113.7		137.6
Depreciation and amortization										73.8	75.3		65.4
Total assets	$ 375.7				$ 404.3					375.7	404.3		434.6
Capital expenditures										69.3	68.5		74.1
Technology | Intersegment
Segment Reporting Information [Line Items]
Revenue										22.6	48.9		58.1
Technology | Operating Segments
Segment Reporting Information [Line Items]
Revenue										$ 437.0	$ 458.4		$ 628.8

[1] Changed to conform to the current-year presentation. See Note 5.