Accumulated other comprehensive income (loss) - Amounts in OCI (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ (2.2)			$ 11.8			$ (139.3)	$ 41.1
Amounts Arising During the Current Period, Income Tax	0.2			1.0			0.5	1.1
Amounts Reclassified to Net Income (Loss), Pretax	14.3			13.6			159.2	39.6
Amounts Reclassified to Net Income (Loss), Income Tax	(3.3)			(4.7)			(10.6)	(13.6)
Other comprehensive income	9.0	$ (11.4)	$ 12.2	21.7	$ 12.2	$ 34.3	9.8	68.2
Cost of revenues	652.6			666.4			2,013.0	1,905.6
Selling, general and administrative expenses	125.4			116.6			368.4	346.5
Interest and other income (expense)	(8.1)			(21.2)			(29.3)	(43.8)
Other operating income (expense)	7.4			0.1			6.3	6.1
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.1)			(5.0)			(0.8)	(9.3)
Amounts Arising During the Current Period, Income Tax	0.2			1.1			0.7	1.5
Amounts Reclassified to Net Income (Loss), Pretax	13.8			14.0			51.5	39.8
Amounts Reclassified to Net Income (Loss), Income Tax	(3.3)			(4.9)			(10.1)	(13.9)
Other comprehensive income	9.6			5.2			41.3	18.1
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.6)			14.6			(138.7)	48.5
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			107.2	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			(0.5)	0.0
Other comprehensive income	(0.6)			14.6			(32.0)	48.5
Unrealized gains (losses) on available-for-sale securities
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax				0.4				1.3
Amounts Arising During the Current Period, Income Tax				(0.1)				(0.4)
Amounts Reclassified to Net Income (Loss), Pretax				(0.7)				(0.9)
Amounts Reclassified to Net Income (Loss), Income Tax				0.2				0.3
Other comprehensive income				(0.2)			0.0	0.3
Gains (losses) on cash flow hedges
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.1			(0.1)			0.7	(0.3)
Amounts Arising During the Current Period, Income Tax	0.0			0.0			(0.2)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	(0.1)			0.1			(0.1)	0.2
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			0.0	0.0
Other comprehensive income	0.0			0.0			0.4	(0.1)
Gains (losses) on cash flow hedges | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Interest and other income (expense)	0.0			0.0			0.0	(0.1)
Other operating income (expense)	0.0			0.1			0.0	0.1
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.6)			9.9			(138.8)	40.2
Amounts Arising During the Current Period, Income Tax	0.2			1.0			0.5	1.1
Amounts Reclassified to Net Income (Loss), Pretax	13.7			13.4			158.6	39.1
Amounts Reclassified to Net Income (Loss), Income Tax	(3.3)			(4.7)			(10.6)	(13.6)
Other comprehensive income	9.0	$ (10.4)	$ 11.1	19.6	$ 14.0	$ 33.2	9.7	66.8
Benefit plan adjustments
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	0.0			0.2			0.0	0.5
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			0.0	0.0
Other comprehensive income	0.0			0.2			0.0	0.5
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.6)			1.9			(0.5)	0.9
Amounts Arising During the Current Period, Income Tax	0.0			0.0			0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.6			0.0			0.6	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			0.0	0.0
Other comprehensive income	0.0			1.9			0.1	0.9
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.6)			1.9			(0.5)	0.9
Amounts Arising During the Current Period, Income Tax	0.0			0.0			0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.6			0.2			0.6	0.5
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			0.0	0.0
Other comprehensive income	0.0			2.1			0.1	1.4
Benefit plan adjustments(a)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.1)			(5.0)			(0.8)	(9.3)
Amounts Arising During the Current Period, Income Tax	0.2			1.1			0.7	1.5
Amounts Reclassified to Net Income (Loss), Pretax	13.8			14.2			51.5	40.3
Amounts Reclassified to Net Income (Loss), Income Tax	(3.3)			(4.9)			(10.1)	(13.9)
Other comprehensive income	9.6			5.4			41.3	18.6
Benefit plan adjustments(a) | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	1.9			2.2			6.4	6.9
Selling, general and administrative expenses	0.7			0.6			1.9	1.7
Interest and other income (expense)	8.9			11.7			30.6	35.4
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.2)			16.5			(139.2)	49.4
Amounts Arising During the Current Period, Income Tax	0.0			0.0			0.0	0.0
Amounts Reclassified to Net Income (Loss), Pretax	0.6			0.0			107.8	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			(0.5)	0.0
Other comprehensive income	(0.6)			16.5			(31.9)	49.4
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
Amounts Arising During the Current Period, Income Tax				(0.1)				(0.4)
Amounts Reclassified to Net Income (Loss), Pretax				(0.7)				(0.9)
Amounts Reclassified to Net Income (Loss), Income Tax				0.2				0.3
Other comprehensive income				(0.2)				0.3
Gains (losses) on cash flow hedges(c)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	0.1			(0.1)			0.7	(0.3)
Amounts Arising During the Current Period, Income Tax	0.0			0.0			(0.2)	0.0
Amounts Reclassified to Net Income (Loss), Pretax	(0.1)			0.1			(0.1)	0.2
Amounts Reclassified to Net Income (Loss), Income Tax	0.0			0.0			0.0	0.0
Other comprehensive income	$ 0.0			$ 0.0			$ 0.4	$ (0.1)