Benefit Costs Included in Accumulated Other Comprehensive Loss that have not yet Recognized in Net Periodic Pension Cost (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost, accumulated taxes	$ 0.9	$ 1.3
Net actuarial loss, accumulated taxes	165.6	130.6
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service cost, accumulated taxes	(0.4)	(0.4)
Net actuarial loss, accumulated taxes	$ 6.0	$ 7.1