Accumulated other comprehensive income (loss) - Amounts in OCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ 33.0		$ (44.5)		$ 67.0	$ (13.4)
Amounts Arising During the Current Period, Income Tax	(0.8)		0.7		2.3	2.6
Amounts Reclassified to Net Income (Loss), Pretax	(1.7)		4.9		(2.8)	28.1
Amounts Reclassified to Net Income (Loss), Income Tax	(0.2)		(0.7)		(0.2)	(7.5)
Other comprehensive income (loss)	30.3	$ 36.0	(39.6)	$ 49.4	66.3	9.8
Cost of revenues	943.8		867.5		1,864.1	1,707.2
Selling, general and administrative expenses	170.6		167.5		347.6	339.1
Interest and other income (expense)	4.1		3.4		8.8	2.1
Other operating income (expense)	(3.8)		2.4		4.5	2.7
Interest expense	51.1		32.4		97.7	60.3
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(3.2)		1.9		(3.7)	1.0
Amounts Arising During the Current Period, Income Tax	0.6		(0.1)		0.5	0.1
Amounts Reclassified to Net Income (Loss), Pretax	0.8		9.6		1.4	19.7
Amounts Reclassified to Net Income (Loss), Income Tax	(0.3)		(2.3)		(0.4)	(4.7)
Other comprehensive income (loss)	(2.1)		9.1		(2.2)	16.1
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	20.2		(56.1)		64.8	(21.0)
Amounts Arising During the Current Period, Income Tax	1.9		4.8		2.0	4.4
Amounts Reclassified to Net Income (Loss), Pretax	(1.5)		(1.5)		(2.9)	(3.0)
Amounts Reclassified to Net Income (Loss), Income Tax	0.4		0.3		0.7	0.7
Other comprehensive income (loss)	21.0		(52.5)		64.6	(18.9)
Unrealized gains (losses) on available-for-sale securities
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	2.3		(0.3)		0.4	(0.7)
Amounts Arising During the Current Period, Income Tax	(0.8)		(0.1)		(0.1)	(0.1)
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 (loss)	1.5		(0.4)		0.3	(0.8)
Gains (losses) on cash flow hedges
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	16.7		15.0		8.3	13.8
Amounts Arising During the Current Period, Income Tax	(2.5)		(3.9)		(0.1)	(1.8)
Amounts Reclassified to Net Income (Loss), Pretax	(1.0)		(3.2)		(1.3)	11.4
Amounts Reclassified to Net Income (Loss), Income Tax	(0.3)		1.3		(0.5)	(3.5)
Other comprehensive income (loss)	12.9		9.2		6.4	19.9
Gains (losses) on cash flow hedges | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Other operating income (expense)	3.8		(5.3)		7.2	6.5
Interest expense	(4.8)		2.1		(8.5)	4.9
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	36.0		(39.5)		69.8	(6.9)
Amounts Arising During the Current Period, Income Tax	(0.8)		0.7		2.3	2.6
Amounts Reclassified to Net Income (Loss), Pretax	(1.7)		4.9		(2.8)	28.1
Amounts Reclassified to Net Income (Loss), Income Tax	(0.2)		(0.7)		(0.2)	(7.5)
Other comprehensive income (loss)	33.3	$ 35.8	(34.6)	$ 50.9	69.1	16.3
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(3.0)		(5.0)		(2.8)	(6.5)
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		0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0		0.0		0.0	0.0
Other comprehensive income (loss)	(3.0)		(5.0)		(2.8)	(6.5)
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(3.0)		(5.0)		(2.8)	(6.5)
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		0.0	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0		0.0		0.0	0.0
Other comprehensive income (loss)	(3.0)		(5.0)		(2.8)	(6.5)
Benefit plan adjustments(a)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(3.2)		1.9		(3.7)	1.0
Amounts Arising During the Current Period, Income Tax	0.6		(0.1)		0.5	0.1
Amounts Reclassified to Net Income (Loss), Pretax	0.8		9.6		1.4	19.7
Amounts Reclassified to Net Income (Loss), Income Tax	(0.3)		(2.3)		(0.4)	(4.7)
Other comprehensive income (loss)	(2.1)		9.1		(2.2)	16.1
Benefit plan adjustments(a) | Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	1.5		1.7		2.9	3.3
Selling, general and administrative expenses	0.5		0.4		1.0	0.9
Interest and other income (expense)	0.1		3.2		0.8	8.0
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	17.2		(61.1)		62.0	(27.5)
Amounts Arising During the Current Period, Income Tax	1.9		4.8		2.0	4.4
Amounts Reclassified to Net Income (Loss), Pretax	(1.5)		(1.5)		(2.9)	(3.0)
Amounts Reclassified to Net Income (Loss), Income Tax	0.4		0.3		0.7	0.7
Other comprehensive income (loss)	18.0		(57.5)		61.8	(25.4)
Unrealized gains (losses) on available-for-sale securities(c)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	2.3		(0.3)		0.4	(0.7)
Amounts Arising During the Current Period, Income Tax	(0.8)		(0.1)		(0.1)	(0.1)
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 (loss)	1.5		(0.4)		0.3	(0.8)
Gains (losses) on cash flow hedges(d)
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	16.7		15.0		8.3	13.8
Amounts Arising During the Current Period, Income Tax	(2.5)		(3.9)		(0.1)	(1.8)
Amounts Reclassified to Net Income (Loss), Pretax	(1.0)		(3.2)		(1.3)	11.4
Amounts Reclassified to Net Income (Loss), Income Tax	(0.3)		1.3		(0.5)	(3.5)
Other comprehensive income (loss)	$ 12.9		$ 9.2		$ 6.4	$ 19.9