Reclassifications out of Accumulated Other Comprehensive Loss - Changes in Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 1,038.0
Other comprehensive income/(losses) before reclassifications	15.9	$ (29.2)
Amounts reclassified from AOCL	(1.3)	0.8
Actuarial net gains/(losses) arising during the year	(17.5)	(69.9)	$ (28.2)
Net current period other comprehensive income/(losses)	(2.9)	(98.3)
Ending Balance	1,147.1	1,038.0
Accumulated Other Comprehensive Loss [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(145.2)	(46.9)
Actuarial net gains/(losses) arising during the year	(17.5)	(69.9)	(28.2)
Ending Balance	(148.1)	(145.2)	(46.9)
Defined Benefit Pension Plan Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(58.4)	10.7
Other comprehensive income/(losses) before reclassifications	0.0	0.0
Amounts reclassified from AOCL	(1.3)	0.8
Actuarial net gains/(losses) arising during the year	(17.5)	(69.9)
Net current period other comprehensive income/(losses)	(18.8)	(69.1)
Ending Balance	(77.2)	(58.4)	10.7
Foreign Currency Translation Items [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(86.8)	(57.6)
Other comprehensive income/(losses) before reclassifications	15.9	(29.2)
Amounts reclassified from AOCL	0.0	0.0
Actuarial net gains/(losses) arising during the year	0.0	0.0
Net current period other comprehensive income/(losses)	15.9	(29.2)
Ending Balance	$ (70.9)	$ (86.8)	$ (57.6)