Employee Benefits Plans (Detail) - Assumptions used	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Employee Retirement Plans, Defined Benefit [Member]
Discount rate for funded status	3.10%	4.15%	4.90%
Discount rate for benefit cost	4.15%	4.90%	5.50%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Discount rate for funded status	2.65%	3.70%	4.35%
Discount rate for benefit cost	3.70%	4.35%	4.75%
Health care cost assumed trend rate for next year	8.00%	8.50%	9.00%
Rate that the cost trend rate gradually declines to	5.50%	5.50%	5.50%
Year that the rate reaches the rate it is assumed to remain at	2018	2018	2018