Accumulated other comprehensive (loss) - Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ 11.8	$ (3.6)
Amounts Arising During the Current Period, Income Tax	1.0	0.1
Amounts Reclassified to Net Income (Loss), Pretax	13.6	12.7
Amounts Reclassified to Net Income (Loss), Income Tax	(4.7)	(4.2)
Other comprehensive income	21.7	5.0	$ 68.2	$ 34.2
Cost of revenues	666.4	594.4	1,905.6	1,779.4
Selling, general and administrative expenses	116.6	102.2	346.5	315.9
Interest and other income (expense)	(21.2)	(9.2)	(43.8)	(28.3)
Other operating income (expense)	0.1	(0.5)	6.1	6.4
Benefit plan adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(5.0)	(0.7)	(9.3)	(2.6)
Amounts Arising During the Current Period, Income Tax	1.1	0.1	1.5	0.6
Amounts Reclassified to Net Income (Loss), Pretax	14.0	12.5	39.8	37.4
Amounts Reclassified to Net Income (Loss), Income Tax	(4.9)	(4.1)	(13.9)	(12.8)
Other comprehensive income	5.2	7.8	18.1	22.6
Foreign currency translation adjustments, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	14.6	(3.4)	48.5	10.2
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	14.6	(3.4)	48.5	10.2
Unrealized gains (losses) on available-for-sale securities, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.4		1.3	0.5
Amounts Arising During the Current Period, Income Tax	(0.1)		(0.4)	(0.2)
Amounts Reclassified to Net Income (Loss), Pretax	(0.7)		(0.9)	(0.5)
Amounts Reclassified to Net Income (Loss), Income Tax	0.2		0.3	0.2
Other comprehensive income	(0.2)		0.3	0.0
Gains (losses) on cash flow hedges, Attributable to parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.1)	0.1	(0.3)	(2.1)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.5
Amounts Reclassified to Net Income (Loss), Pretax	0.1	0.1	0.2	1.9
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	(0.1)	0.0	(0.4)
Other comprehensive income	0.0	0.1	(0.1)	(0.1)
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	0.0	(0.1)	(0.2)
Other operating income (expense)	0.1	(0.1)	0.1	1.3
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	9.9	(4.0)	40.2	6.0
Amounts Arising During the Current Period, Income Tax	1.0	0.1	1.1	0.9
Amounts Reclassified to Net Income (Loss), Pretax	13.4	12.6	39.1	38.8
Amounts Reclassified to Net Income (Loss), Income Tax	(4.7)	(4.2)	(13.6)	(13.0)
Other comprehensive income	19.6	4.5	66.8	32.7
Benefit plan adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.0	0.0	0.0	0.0
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.1	0.5	0.4
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	0.2	0.1	0.5	0.4
Foreign currency translation adjustments, Attributable to noncontrolling interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	1.9	0.4	0.9	1.1
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	1.9	0.4	0.9	1.1
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	1.9	0.4	0.9	1.1
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.2	0.1	0.5	0.4
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	2.1	0.5	1.4	1.5
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(5.0)	(0.7)	(9.3)	(2.6)
Amounts Arising During the Current Period, Income Tax	1.1	0.1	1.5	0.6
Amounts Reclassified to Net Income (Loss), Pretax	14.2	12.6	40.3	37.8
Amounts Reclassified to Net Income (Loss), Income Tax	(4.9)	(4.1)	(13.9)	(12.8)
Other comprehensive income	5.4	7.9	18.6	23.0
Benefit plan adjustments | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	2.2	2.1	6.9	6.5
Selling, general and administrative expenses	0.6	0.5	1.7	1.6
Interest and other income (expense)	11.7	9.7	35.4	29.5
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	16.5	(3.0)	49.4	11.3
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.0	0.0	0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	0.0	0.0	0.0
Other comprehensive income	16.5	(3.0)	49.4	11.3
Unrealized gains (losses) on available-for-sale securities(b)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.4		1.3	0.5
Amounts Arising During the Current Period, Income Tax	(0.1)		(0.4)	(0.2)
Amounts Reclassified to Net Income (Loss), Pretax	(0.7)		(0.9)	(0.5)
Amounts Reclassified to Net Income (Loss), Income Tax	0.2		0.3	0.2
Other comprehensive income	(0.2)		0.3	0.0
Gains (losses) on cash flow hedges(c)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.1)	0.1	(0.3)	(2.1)
Amounts Arising During the Current Period, Income Tax	0.0	0.0	0.0	0.5
Amounts Reclassified to Net Income (Loss), Pretax	0.1	0.1	0.2	1.9
Amounts Reclassified to Net Income (Loss), Income Tax	0.0	(0.1)	0.0	(0.4)
Other comprehensive income	$ 0.0	$ 0.1	(0.1)	(0.1)
AOCI Including Portion Attributable to Noncontrolling Interest [Member]
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax			41.1	7.1
Amounts Arising During the Current Period, Income Tax			1.1	0.9
Amounts Reclassified to Net Income (Loss), Pretax			39.6	39.2
Amounts Reclassified to Net Income (Loss), Income Tax			(13.6)	(13.0)
Other comprehensive income			$ 68.2	$ 34.2