Allocation of Plan Assets by Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 4,094.5	$ 3,793.4	$ 3,226.4
Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	83.0	86.5
Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	427.8	426.2
Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	510.8	512.7
MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	3,583.7	3,280.7
U.S. Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,125.7	1,047.0
U.S. Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,125.7	1,047.0
Global Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,240.8	1,140.6
Global Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,240.8	1,140.6
Total Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 2,366.5	$ 2,187.6
Total Equities | Minimum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	50.00%	50.00%
Total Equities | Maximum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	70.00%	70.00%
Total Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 2,366.5	$ 2,187.6
U.S. Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,182.8	1,099.8
U.S. Fixed Income | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	75.9	83.9
U.S. Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	303.0	296.0
U.S. Fixed Income | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	378.9	379.9
U.S. Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	803.9	719.9
Non-U.S. Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	447.3	403.2
Non-U.S. Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	35.1	30.4
Non-U.S. Fixed Income | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	35.1	30.4
Non-U.S. Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	412.2	372.8
Total Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,630.1	$ 1,503.0
Total Fixed Income | Minimum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	30.00%	30.00%
Total Fixed Income | Maximum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	50.00%	50.00%
Total Fixed Income | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 75.9	$ 83.9
Total Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	338.1	326.4
Total Fixed Income | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	414.0	410.3
Total Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,216.1	1,092.7
Cash and Other
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	97.9	102.8
Cash and Other | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	7.1	2.6
Cash and Other | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	89.7	99.8
Cash and Other | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	96.8	102.4
Cash and Other | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1.1	$ 0.4