Annual report pursuant to Section 13 and 15(d)

Weighted-Average Assumptions used to Determine Net Periodic Benefit Cost (Detail)

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Weighted-Average Assumptions used to Determine Net Periodic Benefit Cost (Detail)
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Pension Benefits
     
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.60% 5.24% 5.77%
Expected return on plan assets 7.67% 7.73% 7.73%
Rate of compensation increase 4.41% 4.37% 4.37%
Other Benefits
     
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 4.29% 4.90% 5.45%
Expected return on plan assets 7.75% 7.75% 7.75%
Rate of compensation increase