Allocation of Plan Assets by Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 3,793.4	$ 3,226.4	$ 2,699.2
Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	86.5	101.2
Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	426.2	356.6
Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	512.7	457.8
MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	3,280.7	2,768.6
U.S. Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,047.0	860.1
U.S. Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,047.0	860.1
Global Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,140.6	973.1
Global Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,140.6	973.1
Total Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 2,187.6	$ 1,833.2
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	$ 2,187.6	$ 1,833.2
U.S. Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,099.8	962.6
U.S. Fixed Income | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	83.9	90.6
U.S. Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	296.0	260.3
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	379.9	350.9
U.S. Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	719.9	611.7
Non-U.S. Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	403.2	350.1
Non-U.S. Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	30.4	28.1
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	30.4	28.1
Non-U.S. Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	372.8	322.0
Total Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,503.0	$ 1,312.7
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	$ 83.9	$ 90.6
Total Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	326.4	288.4
Total Fixed Income | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	410.3	379.0
Total Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	1,092.7	933.7
Cash and Other
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	102.8	80.5
Cash and Other | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	2.6	10.6
Cash and Other | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	99.8	68.2
Cash and Other | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	102.4	78.8
Cash and Other | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 0.4	$ 1.7