Amount of Prior Service Cost and Actuarial Loss Included in Accumulated Other Comprehensive Loss that is Expected to Be Recognized in Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss, net of taxes of $5.8 for pension benefits and $0.4 for other benefits	$ 10.2
Prior service cost, net of taxes of $0.3 for pension benefits and $(0.1) for other benefits	0.5
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss, net of taxes of $5.8 for pension benefits and $0.4 for other benefits	0.6
Prior service cost, net of taxes of $0.3 for pension benefits and $(0.1) for other benefits	$ (0.1)