Allocation of Plan Assets by Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 2,919.6	$ 2,494.1	$ 2,219.0
MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	2,351.4	2,013.1
Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	232.5	204.5
Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	335.7	276.5
Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	568.2	481.0
Equity Funds
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,635.3	$ 1,436.9
Equity Funds | Minimum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	50.00%	50.00%
Equity Funds | Maximum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	70.00%	70.00%
Equity Funds | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,635.3	$ 1,436.9
Equity Funds | United States
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	768.5	690.8
Equity Funds | United States | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	768.5	690.8
Equity Funds | Global
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	866.8	746.1
Equity Funds | Global | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	866.8	746.1
Fixed Income Funds
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 1,204.3	$ 984.5
Fixed Income Funds | Minimum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	30.00%	30.00%
Fixed Income Funds | Maximum
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Equity funds, target	50.00%	50.00%
Fixed Income Funds | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 715.4	$ 572.3
Fixed Income Funds | United States
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	877.5	710.4
Fixed Income Funds | United States | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	416.0	317.8
Fixed Income Funds | Non U.S.
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	326.8	274.1
Fixed Income Funds | Non U.S. | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	299.4	254.5
Fixed Income Funds | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	223.3	195.7
Fixed Income Funds | Level 1 | United States
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	223.3	195.7
Fixed Income Funds | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	265.6	216.5
Fixed Income Funds | Level 2 | United States
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	238.2	196.9
Fixed Income Funds | Level 2 | Non U.S.
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	27.4	19.6
Fixed Income Funds | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	488.9	412.2
Fixed Income Funds | Fair Value Inputs Level 1 And Level 2 [Member] | United States
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	461.5	392.6
Fixed Income Funds | Fair Value Inputs Level 1 And Level 2 [Member] | Non U.S.
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	27.4	19.6
Cash and other
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	80.0	72.7
Cash and other | MEASURED AT NAV
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	0.7	3.9
Cash and other | Level 1
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	9.2	8.8
Cash and other | Level 2
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	70.1	60.0
Cash and other | Fair Value Inputs Level 1 And Level 2 [Member]
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Plan assets	$ 79.3	$ 68.8