Reconciliation of Beginning and Ending Balances for Plan Assets Valued using Significant Unobservable Inputs (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Private Equity
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning balance	$ 31.8
Return on plan assets:
Unrealized	3.2
Purchases	4.1
Sales	(6.1)
Level 3 transfers, net
Ending balance	33.0
Hedge Funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning balance	93.2
Return on plan assets:
Unrealized	(4.7)
Realized	0.1
Purchases	8.7
Sales	(4.4)
Level 3 transfers, net
Ending balance	92.9
Real Assets
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning balance	8.0
Return on plan assets:
Unrealized	0.9
Realized	0.3
Purchases	0.9
Sales	(0.3)
Level 3 transfers, net
Ending balance	$ 9.8