Annual report pursuant to Section 13 and 15(d)

Fair Value of Postretirement Assets (Parenthetical) (Detail)

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Fair Value of Postretirement Assets (Parenthetical) (Detail)
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]  
Hedge funds, minimum number of days notice prior to redemption 30 days
Hedge funds, maximum number of days notice prior to redemption 90 days