Summary of Pre-Tax Amounts Recognized in Other Comprehensive Income (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pension Plans
Schedule of Pension and Other Retirement Benefits Recognized in Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of net actuarial losses	$ 9.1	$ 5.0
Amortization of prior service costs	0.7	0.6
Accelerated recognition of actuarial loss due to settlement		1.6
Net actuarial loss arising during the period	(24.7)	(52.8)
Total recognized in OCI - pre-tax	(14.9)	(45.6)
Other Retirement Plans
Schedule of Pension and Other Retirement Benefits Recognized in Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of net actuarial losses	0.3	0.3
Net actuarial loss arising during the period	(0.2)	(3.3)
Total recognized in OCI - pre-tax	$ 0.1	$ (3.0)