Accumulated Other Comprehensive Income - Changes in Accumulated Other Comprehensive Income/(Loss) Net of Tax by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss) net of tax, beginning balance	$ (271.9)
Accumulated other comprehensive income (loss) net of tax, ending balance	(165.1)	$ (271.9)
Foreign Currency Translation [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss) net of tax, beginning balance	(47.0)	(48.6)
Other comprehensive income (loss) before reclassifications	(2.1)	1.6
Net current-period other comprehensive income (loss)	(2.1)	1.6
Accumulated other comprehensive income (loss) net of tax, ending balance	(49.1)	(47.0)
Benefit Plan Liabilities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss) net of tax, beginning balance	(221.8)	(253.1)
Other comprehensive income (loss) before reclassifications	28.0	(23.4)
Amounts reclassified from accumulated other comprehensive income (loss)	79.3	54.7
Net current-period other comprehensive income (loss)	107.3	31.3
Accumulated other comprehensive income (loss) net of tax, ending balance	(114.5)	(221.8)
Cash Flow Hedge - Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss) net of tax, beginning balance	(3.1)	(0.3)
Other comprehensive income (loss) before reclassifications		(4.1)
Amounts reclassified from accumulated other comprehensive income (loss)	1.6	1.3
Net current-period other comprehensive income (loss)	1.6	(2.8)
Accumulated other comprehensive income (loss) net of tax, ending balance	$ (1.5)	$ (3.1)