Fair Value of Pension Plan Assets (Detail) - Pension Plans, Defined Benefit - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 1,393.5	$ 1,342.0
Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	52.1	45.4
Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.1	0.3
U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	134.6	135.7
Northern Trust Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	70.7	64.3
Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		0.5
Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,029.9	984.9
Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	35.7	47.5
Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	64.8	63.4
Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	5.6
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	128.5	110.5
Fair Value, Inputs, Level 1 | Domestic Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	52.1	45.4
Fair Value, Inputs, Level 1 | Foreign Common Stock
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	0.1	0.3
Fair Value, Inputs, Level 1 | Northern Trust Mutual Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	70.7	64.3
Fair Value, Inputs, Level 1 | Exchange Traded Fund
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value		0.5
Fair Value, Inputs, Level 1 | Cash and Other
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	5.6
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,164.5	1,120.6
Fair Value, Inputs, Level 2 | U.S. Government Obligations
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	134.6	135.7
Fair Value, Inputs, Level 2 | Northern Trust Collective Trust Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	1,029.9	984.9
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	100.5	110.9
Fair Value, Inputs, Level 3 | Northern Trust Private Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	35.7	47.5
Fair Value, Inputs, Level 3 | Northern Trust Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Fair Value	$ 64.8	$ 63.4