Accumulated other comprehensive income (loss) - Amounts in OCI (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	$ 82.2		$ (70.5)		$ 118.5	$ (74.8)
Amounts Arising During the Current Period, Income Tax	8.2		(2.0)		11.9	(9.5)
Amounts Reclassified to Net Income (Loss), Pretax	2.4		(4.5)		0.1	(8.5)
Amounts Reclassified to Net Income (Loss), Income Tax	0.4		1.1		1.2	2.1
Other comprehensive income (loss)	93.2	$ 38.5	(75.9)	$ (14.8)	131.7	(90.7)
Cost of revenues	976.7		937.8		1,916.2	1,865.0
Selling, general and administrative expenses	184.5		194.3		370.8	394.9
Interest and other nonoperating expense	(1.8)		(12.5)		(9.7)	(25.8)
Interest expense	60.9		56.5		118.4	112.3
AOCI Attributable to Parent
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	78.3		(69.6)		113.9	(72.1)
Amounts Arising During the Current Period, Income Tax	8.2		(2.0)		11.9	(9.5)
Amounts Reclassified to Net Income (Loss), Pretax	2.4		(4.5)		0.1	(8.5)
Amounts Reclassified to Net Income (Loss), Income Tax	0.4		1.1		1.2	2.1
Other comprehensive income (loss)	89.3	$ 37.8	(75.0)	$ (13.0)	127.1	(88.0)
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(4.4)		(6.8)		(7.8)	(7.5)
Amounts Arising During the Current Period, Income Tax	1.3		1.4		2.2	1.5
Amounts Reclassified to Net Income (Loss), Pretax	1.6		1.4		2.8	3.5
Amounts Reclassified to Net Income (Loss), Income Tax	(0.4)		(0.3)		(0.7)	(0.8)
Other comprehensive income (loss)	(1.9)		(4.3)		(3.5)	(3.3)
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	84.8		(69.0)		123.5	(87.6)
Amounts Arising During the Current Period, Income Tax	8.0		(1.6)		10.9	(3.8)
Amounts Reclassified to Net Income (Loss), Pretax	(1.2)		(1.2)		(2.4)	(2.3)
Amounts Reclassified to Net Income (Loss), Income Tax	0.3		0.2		0.6	0.5
Other comprehensive income (loss)	91.9		(71.6)		132.6	(93.2)
Gains (losses) on available-for-sale securities
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.8)		0.6		(1.4)	1.7
Amounts Arising During the Current Period, Income Tax	(1.2)		(0.4)		(1.3)	(2.3)
Amounts Reclassified to Net Income (Loss), Pretax	4.1		0.0		4.3	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0		0.0		0.2	0.0
Other comprehensive income (loss)	1.1		0.2		1.8	(0.6)
Gains (Losses) on Cash Flow Hedges
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.3)		5.6		(0.4)	21.3
Amounts Arising During the Current Period, Income Tax	0.1		(1.4)		0.1	(4.9)
Amounts Reclassified to Net Income (Loss), Pretax	(2.1)		(4.7)		(4.6)	(9.7)
Amounts Reclassified to Net Income (Loss), Income Tax	0.5		1.2		1.1	2.4
Other comprehensive income (loss)	(1.8)		0.7		(3.8)	9.1
Gains (Losses) on Cash Flow Hedges \| Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Interest expense	(2.1)		(4.7)		(4.6)	(9.7)
AOCI Attributable to Noncontrolling Interest
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	3.9		(0.9)		4.6	(2.7)
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)	3.9		(0.9)		4.6	(2.7)
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	3.9		(0.9)		4.6	(2.7)
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)	3.9		(0.9)		4.6	(2.7)
Benefit plan adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(4.4)		(6.8)		(7.8)	(7.5)
Amounts Arising During the Current Period, Income Tax	1.3		1.4		2.2	1.5
Amounts Reclassified to Net Income (Loss), Pretax	1.6		1.4		2.8	3.5
Amounts Reclassified to Net Income (Loss), Income Tax	(0.4)		(0.3)		(0.7)	(0.8)
Other comprehensive income (loss)	(1.9)		(4.3)		(3.5)	(3.3)
Benefit plan adjustments \| Reclassification out of accumulated other comprehensive income
Other Comprehensive Income Loss [Line Items]
Cost of revenues	1.7		1.7		3.3	3.4
Selling, general and administrative expenses	0.5		0.5		1.0	1.1
Interest and other nonoperating expense	0.7		0.3		0.8	1.4
Foreign currency translation adjustments
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	88.7		(69.9)		128.1	(90.3)
Amounts Arising During the Current Period, Income Tax	8.0		(1.6)		10.9	(3.8)
Amounts Reclassified to Net Income (Loss), Pretax	(1.2)		(1.2)		(2.4)	(2.3)
Amounts Reclassified to Net Income (Loss), Income Tax	0.3		0.2		0.6	0.5
Other comprehensive income (loss)	95.8		(72.5)		137.2	(95.9)
Gains (losses) on available-for-sale securities
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(1.8)		0.6		(1.4)	1.7
Amounts Arising During the Current Period, Income Tax	(1.2)		(0.4)		(1.3)	(2.3)
Amounts Reclassified to Net Income (Loss), Pretax	4.1		0.0		4.3	0.0
Amounts Reclassified to Net Income (Loss), Income Tax	0.0		0.0		0.2	0.0
Other comprehensive income (loss)	1.1		0.2		1.8	(0.6)
Gains (losses) on cash flow hedges
Other Comprehensive Income Loss [Line Items]
Amounts Arising During the Current Period, Pretax	(0.3)		5.6		(0.4)	21.3
Amounts Arising During the Current Period, Income Tax	0.1		(1.4)		0.1	(4.9)
Amounts Reclassified to Net Income (Loss), Pretax	(2.1)		(4.7)		(4.6)	(9.7)
Amounts Reclassified to Net Income (Loss), Income Tax	0.5		1.2		1.1	2.4
Other comprehensive income (loss)	$ (1.8)		$ 0.7		$ (3.8)	$ 9.1

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