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Assumed Healthcare Cost Trend Rates (Detail)	12 Months Ended
Assumed Healthcare Cost Trend Rates (Detail)	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Assumed Health Care Cost Trend Rates, Effect of One Percentage Point Change [Line Items]
Ultimate rate to which the cost trend rate is assumed to decline (ultimate trend rate)	5.00%	5.00%	5.00%
Year that the rate reaches the ultimate trend rate	2020	2020	2020
Pre-Age 65
Assumed Health Care Cost Trend Rates, Effect of One Percentage Point Change [Line Items]
Healthcare cost trend rate assumed for the following year	6.90%	7.40%	7.90%
Post-Age 65
Assumed Health Care Cost Trend Rates, Effect of One Percentage Point Change [Line Items]
Healthcare cost trend rate assumed for the following year	7.90%	8.40%	8.90%

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

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