Accumulated other comprehensive (loss) - Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ 25.7	$ 15.9
Amounts Arising During the Current Period, Income Tax	0.1	0.4
Amounts Reclassified to Net Income (Loss), Pretax	13.0	13.1
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.2)
Other comprehensive income	34.3	25.2
Cost of revenues	610.3	588.9
Selling, general and administrative expenses	107.1	108.7
Interest and other income (expense)	(11.2)	(9.7)
Other operating expense	(0.1)	(0.7)
Benefit plan adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.5)	(1.1)
Amounts Arising During the Current Period, Income Tax	0.2	0.3
Amounts Reclassified to Net Income (Loss), Pretax	12.6	12.3
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.2)
Other comprehensive income	6.8	7.3
Foreign currency translation adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	26.3	17.1
Amounts Arising During the Current Period, Income Tax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	26.3	17.1
Unrealized gains (losses) on available-for-sale securities, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.2	0.2
Amounts Arising During the Current Period, Income Tax	(0.1)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	0.1	0.2
Gains (losses) on cash flow hedges, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.2)	(1.0)
Amounts Arising During the Current Period, Income Tax	0.0	0.1
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.7
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	0.0	(0.2)
Gains (losses) on cash flow hedges, Attributable to parent | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Interest and other income (expense)	0.0	(0.1)
Other operating expense	(0.2)	0.6
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	24.8	15.2
Amounts Arising During the Current Period, Income Tax	0.1	0.4
Amounts Reclassified to Net Income (Loss), Pretax	12.8	13.0
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.2)
Other comprehensive income	33.2	24.4
Benefit plan adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	0.0
Amounts Arising During the Current Period, Income Tax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.1
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	0.2	0.1
Foreign currency translation adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.9	0.7
Amounts Arising During the Current Period, Income Tax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	0.9	0.7
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.9	0.7
Amounts Arising During the Current Period, Income Tax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.1
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	1.1	0.8
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.5)	(1.1)
Amounts Arising During the Current Period, Income Tax	0.2	0.3
Amounts Reclassified to Net Income (Loss), Pretax	12.8	12.4
Amounts Reclassified to Net Income (Loss), Income Tax	(4.5)	(4.2)
Other comprehensive income	7.0	7.4
Benefit plan adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	2.3	2.3
Selling, general and administrative expenses	0.6	0.5
Interest and other income (expense)	11.6	9.7
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	27.2	17.8
Amounts Arising During the Current Period, Income Tax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	27.2	17.8
Unrealized losses on available-for-sale securities(b)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.2	0.2
Amounts Arising During the Current Period, Income Tax	(0.1)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	0.1	0.2
Gains (losses) on cash flow hedges(c)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.2)	(1.0)
Amounts Arising During the Current Period, Income Tax	0.0	0.1
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.7
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0
Other comprehensive income	$ 0.0	$ (0.2)