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Retirement Plans - Actuarial Assumptions Used by Corporation's Postretirement Medical Plan (Detail) (Postretirement Medical Plan [Member])	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Postretirement Medical Plan [Member]
Assumptions used to determine benefit obligations at December 31
Discount rate	3.10%	3.90%	4.80%
Initial health care trend rate	7.30%	8.00%	8.00%
Ultimate trend rate	4.80%	5.00%	5.00%
Year in which ultimate trend rate is reached	2022	2018	2017

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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