Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
LEVEL 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 0.0	$ 0.0
LEVEL 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	2.0
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,601.2	1,380.1
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	12.3	27.7
Pension Plans, Defined Benefit | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	254.6	227.6
Pension Plans, Defined Benefit | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	45.0	34.2
Pension Plans, Defined Benefit | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	168.3	155.5
Pension Plans, Defined Benefit | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	18.8	15.1
Pension Plans, Defined Benefit | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	23.1	22.7
Pension Plans, Defined Benefit | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.3	2.0
Pension Plans, Defined Benefit | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	866.6	745.8
Pension Plans, Defined Benefit | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	112.8	92.4
Pension Plans, Defined Benefit | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		0.1
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	20.3	25.5
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	30.2	29.2
Pension Plans, Defined Benefit | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	46.3
Pension Plans, Defined Benefit | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	2.6	2.3
Pension Plans, Defined Benefit | LEVEL 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	127.7	122.5
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	12.3	27.7
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	112.8	92.4
Pension Plans, Defined Benefit | LEVEL 1 | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		0.1
Pension Plans, Defined Benefit | LEVEL 1 | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0
Pension Plans, Defined Benefit | LEVEL 1 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	2.6	2.3
Pension Plans, Defined Benefit | LEVEL 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,376.7	1,202.9
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	254.6	227.6
Pension Plans, Defined Benefit | LEVEL 2 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	45.0	34.2
Pension Plans, Defined Benefit | LEVEL 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	168.3	155.5
Pension Plans, Defined Benefit | LEVEL 2 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	18.8	15.1
Pension Plans, Defined Benefit | LEVEL 2 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	23.1	22.7
Pension Plans, Defined Benefit | LEVEL 2 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	866.6	745.8
Pension Plans, Defined Benefit | LEVEL 2 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		0.0
Pension Plans, Defined Benefit | LEVEL 2 | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	96.8	54.7
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Non-U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Domestic Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Foreign Municipal and Provincial Bonds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Mutual Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		0.0
Pension Plans, Defined Benefit | LEVEL 3 | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	20.3	25.5	$ 29.3
Pension Plans, Defined Benefit | LEVEL 3 | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	30.2	29.2	44.6
Pension Plans, Defined Benefit | LEVEL 3 | Real Estate Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	46.3	0.0	$ 0.0
Pension Plans, Defined Benefit | LEVEL 3 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 0.0	$ 0.0