Allocation of Plan Assets by Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 2,699.2	$ 2,919.6	$ 2,494.1
Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	232.2	232.5
Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	314.0	335.7
Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	546.2	568.2
MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	2,153.0	2,351.4
U.S. Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	680.2	768.5
U.S. Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	680.2	768.5
Global Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	772.6	866.8
Global Equities | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	772.6	866.8
Total Equities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,452.8	$ 1,635.3
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	$ 1,452.8	$ 1,635.3
U.S. Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	866.2	877.5
U.S. Fixed Income | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	223.2	223.3
U.S. Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	223.4	238.2
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	446.6	461.5
U.S. Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	419.6	416.0
Non-U.S. Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	300.6	326.8
Non-U.S. Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	21.6	27.4
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	21.6	27.4
Non-U.S. Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	279.0	299.4
Total Fixed Income
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,166.8	$ 1,204.3
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	$ 223.2	$ 223.3
Total Fixed Income | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	245.0	265.6
Total Fixed Income | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	468.2	488.9
Total Fixed Income | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	698.6	715.4
Cash and Other
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	79.6	80.0
Cash and Other | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	9.0	9.2
Cash and Other | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	69.0	70.1
Cash and Other | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	78.0	79.3
Cash and Other | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1.6	$ 0.7