Retirement Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012 Employee	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation for all defined benefit pension plans	$ 160.7	$ 143.0
Percentage of excess gains and losses over benefit obligation or market value of assets amortized	10.00%
Costs of defined contribution plans	7.9	7.8	7.0
Number of participants covering under multi employer pension plan	400
Costs of bargaining unit-sponsored multi-employer plans and defined contribution plans	1.0	1.0	0.9
Weighted average annual rate of increase in the per capita cost of covered health care benefits assumed	7.46%
Assumed decrease of weighted average health care cost trend rate	4.49%
Year that reaches assumed decrease of weighted average health care cost trend rate	2027
Increase in service and interest cost components of the net periodic benefit cost, as 1% increase in health care cost trend rates	0.2
Increase in accumulated postretirement benefit obligation, as 1% increase in health care cost trend rates	2.0
Decrease in service and interest cost components of the net periodic benefit cost, as 1% decrease in health care cost trend rates	0.2
Decrease in accumulated postretirement benefit obligation, as 1% decrease in health care cost trend rates	1.7
Minimum fund performance period of plan assets	3
Maximum fund performance period of plan assets	5
Future estimated benefit payments for year 2013	12.8
Future estimated benefit payments for year 2014	10.2
Future estimated benefit payments for year 2015	9.8
Future estimated benefit payments for year 2016	9.9
Future estimated benefit payments for year 2017	10.4
Future estimated benefit payments for years 2018 to 2022	56.7
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year	7.0
Other Postretirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year	0.9
Defined Contribution Plan
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year	8.6
Defined Contribution Pension Plan 401k
Defined Benefit Plan Disclosure [Line Items]
Expected contribution for next fiscal year	$ 7.0
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities, minimum	55.00%
Target allocation of fund assets in equities, maximum	75.00%
Target allocation of fund assets in equities	66.00%
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Target allocation of fund assets in equities, minimum	25.00%
Target allocation of fund assets in equities, maximum	45.00%
Target allocation of fund assets in equities	34.00%