Weighted-Average Assumptions used in Measurement of Benefit Obligations and Net Periodic Benefit Expense (Detail)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Expected return on plan assets	6.48%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit Obligation, Discount rate	5.00%	5.66%
Expected return on plan assets	6.48%	6.56%
Rate of compensation increase	4.22%	4.19%
Net periodic benefit expense, Discount rate	5.50%	5.88%
Net periodic benefit expense, Rate of compensation increase	4.22%	4.19%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit Obligation, Discount rate	4.87%	5.66%
Expected return on plan assets	0.00%	0.00%
Rate of compensation increase	0.00%	0.00%
Net periodic benefit expense, Discount rate	5.50%	5.90%
Net periodic benefit expense, Rate of compensation increase	0.00%	0.00%