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Reconciliation of Beginning and Ending Balances for Plan Assets Valued using Significant Unobservable Inputs (Detail)

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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, 2012
Private Equity
  
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]  
Balance at December 31, 2011 $ 33.0
Return on plan assets:  
Unrealized 1.0
Realized 1.6
Purchases 6.5
Sales (6.1)
Balance at December 31, 2012 36.0
Hedge Funds
  
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]  
Balance at December 31, 2011 92.9
Return on plan assets:  
Unrealized 4.7
Realized 1.0
Purchases 5.9
Sales (15.4)
Balance at December 31, 2012 89.1
Real Assets
  
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]  
Balance at December 31, 2011 9.8
Return on plan assets:  
Unrealized 0.6
Realized 0
Purchases 1.2
Sales (0.5)
Balance at December 31, 2012 $ 11.1