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Employee Benefits - Assumptions Regarding Healthcare Cost Trend Rates (Detail)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]    
Rate to which the cost trend rate is assumed to decline 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2019 2019
Under age 65 [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Healthcare cost trend rate assumed for the next year 8.00% 8.50%
Age 65 and over [Member]
   
Defined Benefit Plan Disclosure [Line Items]    
Healthcare cost trend rate assumed for the next year