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Effect of a One Percentage Point Change in Assumed Health Care Cost Trend Rates (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]  
1-Percentage Point Increase effect on service and interest cost $ 138
1-Percentage Point Increase effect on postretirement benefit obligation 1,740
1-Percentage Point Decrease effect on service and interest cost (118)
1-Percentage Point Decrease effect on postretirement benefit obligation $ (1,524)