Benefit Costs Included in Accumulated Other Comprehensive Loss that have not yet Recognized in Net Periodic Pension Cost (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]
Accumulated other comprehensive loss	$ 300.3	$ 241.3	$ 216.2
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost, net of accumulated taxes of $0.9 and $1.3 in 2011 and 2010, respectively, for pension benefits and $(0.4) in 2011 and 2010 for other benefits	1.8	2.2
Net actuarial loss, net of accumulated taxes of $165.6 and $130.6 in 2011 and 2010, respectively, for pension benefits and $6.0 and $7.1 in 2011 and 2010, respectively, for other benefits	288.6	227.5
Accumulated other comprehensive loss	290.4	229.7
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost, net of accumulated taxes of $0.9 and $1.3 in 2011 and 2010, respectively, for pension benefits and $(0.4) in 2011 and 2010 for other benefits	(0.6)	(0.8)
Net actuarial loss, net of accumulated taxes of $165.6 and $130.6 in 2011 and 2010, respectively, for pension benefits and $6.0 and $7.1 in 2011 and 2010, respectively, for other benefits	10.5	12.4
Accumulated other comprehensive loss	$ 9.9	$ 11.6