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Actuarial Assumptions Used To Determine Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Qualified Defined Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.00%	4.75%	5.50%
Expected long-term rate of return on assets	8.00%	8.00%	8.50%
Rate of increase in future compensation levels	4.30%	4.30%	4.40%
Retiree Medical And Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.75%	4.50%	5.50%
Expected long-term rate of return on assets	8.00%	8.00%	8.50%
Health care trend rate assumed for next year	9.00%	9.50%	10.00%
Ultimate health care trend rate	5.00%	5.00%	5.50%
Year that the ultimate health care trend rate is reached	2029	2021	2021

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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