Fair Value Measurements Of Plan Assets (Detail) - Pension Plans, Defined Benefit - USD ($) $ in Millions		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		$ 469.7	[1]	$ 462.6	[1]	$ 455.6
Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	3.4		7.1
Guaranteed Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	8.0		7.1
Private alternative investment - hedge fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	19.6		19.3
Equity Securities | US Large Cap
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	125.7		117.3
Equity Securities | US Small Cap
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	40.6		37.2
Equity Securities | Foreign Equity Security
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	118.0		117.3
Fixed Income Securities | Diversified Bond Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1],[2]	131.6		123.2
Fixed Income Securities | Foreign Government Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	11.5		21.4
Fixed Income Securities | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[1]	11.3		12.7
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		442.1		430.2
Fair Value, Inputs, Level 1 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		3.4		7.1
Fair Value, Inputs, Level 1 | Guaranteed Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		0.0
Fair Value, Inputs, Level 1 | Private alternative investment - hedge fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		0.0
Fair Value, Inputs, Level 1 | Equity Securities | US Large Cap
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		125.7		117.3
Fair Value, Inputs, Level 1 | Equity Securities | US Small Cap
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		40.6		37.2
Fair Value, Inputs, Level 1 | Equity Securities | Foreign Equity Security
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		118.0		117.3
Fair Value, Inputs, Level 1 | Fixed Income Securities | Diversified Bond Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[2]	131.6		121.1
Fair Value, Inputs, Level 1 | Fixed Income Securities | Foreign Government Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		11.5		17.5
Fair Value, Inputs, Level 1 | Fixed Income Securities | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		11.3		12.7
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		27.6		32.4
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		0.0
Fair Value, Inputs, Level 2 | Guaranteed Insurance Contracts
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		8.0		7.1
Fair Value, Inputs, Level 2 | Private alternative investment - hedge fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		19.6		19.3
Fair Value, Inputs, Level 2 | Equity Securities | US Large Cap
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		0.0
Fair Value, Inputs, Level 2 | Equity Securities | US Small Cap
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		0.0
Fair Value, Inputs, Level 2 | Equity Securities | Foreign Equity Security
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		0.0
Fair Value, Inputs, Level 2 | Fixed Income Securities | Diversified Bond Funds
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value	[2]	0.0		2.1
Fair Value, Inputs, Level 2 | Fixed Income Securities | Foreign Government Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		0.0		3.9
Fair Value, Inputs, Level 2 | Fixed Income Securities | Foreign Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Plan assets fair value		$ 0.0		$ 0.0

[1] There were no plan assets categorized as Level 3 at December 31, 2016 or 2015.

[2] Diversified bond funds consists of U.S. Treasury bonds, mortgage backed securities, and corporate bonds.