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Long Term Asset Allocation Ranges (Detail)
12 Months Ended
Dec. 31, 2011
Domestic equities
 
Defined Benefit Plan Disclosure [Line Items]  
Equity securities, minimum 20.00%
Equity securities, maximum 25.00%
Global/International equity
 
Defined Benefit Plan Disclosure [Line Items]  
Equity securities, minimum 20.00%
Equity securities, maximum 25.00%
Corporate bonds
 
Defined Benefit Plan Disclosure [Line Items]  
Corporate bonds, minimum 50.00%
Corporate bonds, maximum 60.00%
Liquid reserves
 
Defined Benefit Plan Disclosure [Line Items]  
Liquid reserves, minimum 0.00%
Liquid reserves, maximum 1.00%