Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	0.0	0.0
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,380.1	1,506.4
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	36.3	38.6
Pension Plans, Defined Benefit | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,010.2	1,072.0
Pension Plans, Defined Benefit | Northern Trust Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	39.9	44.5
Pension Plans, Defined Benefit | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.1	0.1
Pension Plans, Defined Benefit | Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	226.5	268.6
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	25.5	29.3
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	29.2	44.6
Pension Plans, Defined Benefit | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	12.4	8.7
Pension Plans, Defined Benefit | LEVEL 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	88.7	91.9
Pension Plans, Defined Benefit | LEVEL 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	36.3	38.6
Pension Plans, Defined Benefit | LEVEL 1 | Foreign Common Stock
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 | Northern Trust Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	39.9	44.5
Pension Plans, Defined Benefit | LEVEL 1 | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.1	0.1
Pension Plans, Defined Benefit | LEVEL 1 | Northern Trust Collective Trust 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 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	12.4	8.7
Pension Plans, Defined Benefit | LEVEL 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,236.7	1,340.6
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 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,010.2	1,072.0
Pension Plans, Defined Benefit | LEVEL 2 | Northern Trust Mutual Fund
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	0.0
Pension Plans, Defined Benefit | LEVEL 2 | Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	226.5	268.6
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 | 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	54.7	73.9
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 | Foreign Common Stock
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 | Northern Trust Mutual Fund
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	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.0
Pension Plans, Defined Benefit | LEVEL 3 | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	25.5	29.3	$ 35.7
Pension Plans, Defined Benefit | LEVEL 3 | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	29.2	44.6	$ 64.8
Pension Plans, Defined Benefit | LEVEL 3 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 0.0	$ 0.0