Employee Benefits - Fair Value of U.S. Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		$ 0.0
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 1,506.4	1,393.5
Pension Plans, Defined Benefit | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	38.6	52.1
Pension Plans, Defined Benefit | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.1
Pension Plans, Defined Benefit | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,072.0	134.6
Pension Plans, Defined Benefit | Northern Trust Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	44.5	70.7
Pension Plans, Defined Benefit | Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	268.6	1,029.9
Pension Plans, Defined Benefit | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	29.3	35.7
Pension Plans, Defined Benefit | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	44.6	64.8
Pension Plans, Defined Benefit | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	8.7	5.6
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	91.9	128.5
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	38.6	52.1
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.0	0.1
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1 | Northern Trust Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	44.5	70.7
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 1 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	8.7	5.6
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,340.6	1,164.5
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,072.0	134.6
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 2 | Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	268.6	1,029.9
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	73.9	100.5
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 3 | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	29.3	35.7
Pension Plans, Defined Benefit | Fair Value, Inputs, Level 3 | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 44.6	$ 64.8