Weighted Average Assumptions used in Determining Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.28%	5.50%	5.78%
Rate of return	8.27%	8.31%	8.27%
Other Benefit
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.95%	5.10%	5.60%
Initial healthcare trend rate	8.50%	8.00%	9.00%
Ultimate healthcare trend rate	5.00%	5.00%	5.00%