Summary of Pension Plan Assets by Category Based on Hierarchy of Fair Value Measurements (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	$ 167.6	$ 133.0
Percent of total assets	100.00%	100.00%
Cash and Cash Equivalents
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	0.2	0.2
Equity Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	100.1	73.8
Percent of total assets	60.00%	56.00%
Equity Securities | Emerging Markets Equity Fund
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	13.3	7.7
Percent of total assets	8.00%	6.00%
Equity Securities | U.S. Large-Cap
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	32.0	26.4
Percent of total assets	19.00%	20.00%
Equity Securities | U.S. Small And Mid-Cap
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	10.7	9.3
Percent of total assets	6.00%	7.00%
Equity Securities | International
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	44.1	30.4
Percent of total assets	27.00%	23.00%
Fixed Income Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	50.4	44.5
Percent of total assets	30.00%	33.00%
Fixed Income Securities | Long-term government/treasury bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	13.8	13.9
Percent of total assets	8.00%	10.00%
Fixed Income Securities | Long-term investment grade corporate bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	17.5	14.9
Percent of total assets	11.00%	11.00%
Fixed Income Securities | U.S. Treasury Inflation-Protected Securities (TIPs)
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	8.5	7.6
Percent of total assets	5.00%	6.00%
Fixed Income Securities | Emerging Markets Bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	5.4	4.5
Percent of total assets	3.00%	3.00%
Fixed Income Securities | High Yield Bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	5.2	3.6
Percent of total assets	3.00%	3.00%
Total Other Investment
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	16.9	14.5
Percent of total assets	10.00%	11.00%
Total Other Investment | Common/Collective Trust Funds - Convertible Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	4.8	4.8
Percent of total assets	3.00%	4.00%
Total Other Investment | Private Real Estate Fund
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	12.1	9.7
Percent of total assets	7.00%	7.00%
Level 1
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	13.3	7.7
Level 1 | Equity Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	13.3	7.7
Level 1 | Equity Securities | Emerging Markets Equity Fund
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	13.3	7.7
Level 2
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	142.2	115.6
Level 2 | Cash and Cash Equivalents
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	0.2	0.2
Level 2 | Equity Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	86.8	66.1
Level 2 | Equity Securities | U.S. Large-Cap
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	32.0	26.4
Level 2 | Equity Securities | U.S. Small And Mid-Cap
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	10.7	9.3
Level 2 | Equity Securities | International
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	44.1	30.4
Level 2 | Fixed Income Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	50.4	44.5
Level 2 | Fixed Income Securities | Long-term government/treasury bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	13.8	13.9
Level 2 | Fixed Income Securities | Long-term investment grade corporate bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	17.5	14.9
Level 2 | Fixed Income Securities | U.S. Treasury Inflation-Protected Securities (TIPs)
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	8.5	7.6
Level 2 | Fixed Income Securities | Emerging Markets Bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	5.4	4.5
Level 2 | Fixed Income Securities | High Yield Bonds
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	5.2	3.6
Level 2 | Total Other Investment
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	4.8	4.8
Level 2 | Total Other Investment | Common/Collective Trust Funds - Convertible Securities
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	4.8	4.8
Level 3
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	12.1	9.7
Level 3 | Total Other Investment
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	12.1	9.7
Level 3 | Total Other Investment | Private Real Estate Fund
Schedule of Pension and Other Retirement Benefits Changes in Benefit Obligation and Fair Value of Plan Assets [Line Items]
Balance	$ 12.1	$ 9.7