Fair Value Measurements Of Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	$ 355.7	[1]	$ 298.7	[1]
Cash and Cash Equivalents
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	5.1	[1]	10.4	[1]
Fixed Income Securities | Guaranteed Insurance Contracts
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	5.8	[1]	5.2	[1]
Equity Securities | US Large Cap
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	94.6	[1]	79.1	[1]
Equity Securities | US Mid Cap
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	28.6	[1]	25.5	[1]
Equity Securities | US Small Cap
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	38.7	[1]	29.5	[1]
Equity Securities | Foreign Equity Security
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	64.3	[1]	48.9	[1]
Diversified Bond Fund | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	98.6	[1],[2]	84.7	[1],[2]
Foreign Government Debt Securities | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	9.8	[1]	7.9	[1]
Foreign Corporate Debt Securities | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	10.2	[1]	7.5	[1]
Fair Value, Inputs, Level 1
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	95.1		75.0
Fair Value, Inputs, Level 1 | Equity Securities | US Mid Cap
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	28.6		25.5
Fair Value, Inputs, Level 1 | Equity Securities | US Small Cap
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	38.7		29.5
Fair Value, Inputs, Level 1 | Equity Securities | Foreign Equity Security
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	27.8		20.0
Fair Value, Inputs, Level 2
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	260.6		223.7
Fair Value, Inputs, Level 2 | Cash and Cash Equivalents
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	5.1		10.4
Fair Value, Inputs, Level 2 | Fixed Income Securities | Guaranteed Insurance Contracts
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	5.8		5.2
Fair Value, Inputs, Level 2 | Equity Securities | US Large Cap
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	94.6		79.1
Fair Value, Inputs, Level 2 | Equity Securities | Foreign Equity Security
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	36.5		28.9
Fair Value, Inputs, Level 2 | Diversified Bond Fund | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	98.6	[2]	84.7	[2]
Fair Value, Inputs, Level 2 | Foreign Government Debt Securities | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	9.8		7.9
Fair Value, Inputs, Level 2 | Foreign Corporate Debt Securities | Fixed Income Securities
Schedule of Pension and Other Postretirment Plan Assets by Fair Value [Line Items]
Plan assets fair value	$ 10.2		$ 7.5

[1] There were no assets categorized as Level 3 at December 31, 2012 and 2011, respectively.

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