| Fair Value of Pension Assets (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified | 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 | 
| Private Equity | |
| Defined Benefit Plan Disclosure [Line Items] | |
| Commitments related to investments | 23.9 | 
| Real Assets | |
| Defined Benefit Plan Disclosure [Line Items] | |
| Commitments related to investments | 4.1 | 
| X | ||||||||||
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- Definition Redemption Notice Period, Lower Limit No definition available. 
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| X | ||||||||||
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- Definition Redemption Notice Period Upper Limit No definition available. 
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| X | ||||||||||
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- Details 
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| X | ||||||||||
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- Definition The floor amount as of the balance sheet date that the entity must expend to satisfy the terms of disclosed arrangements (excluding long-term commitments) in which the entity has agreed to expend funds to procure goods or services from one or more suppliers, other than under a long-term purchase commitment or an unconditional purchase obligation. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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