Pension and Other Postretirement Benefits - Assumptions Used in Determining Benefit Obligations and Net Periodic Benefit Cost Amounts (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Weighted average assumptions for benefit obligations at year end:
Discount rate	3.70%	4.50%
Salary increase	3.90%	3.90%
Weighted average assumptions for net periodic cost for the year:
Discount rate	4.50%	5.10%
Salary increase	3.90%	4.00%
Expected return on assets	6.90%	7.40%
Other Benefits [Member]
Weighted average assumptions for benefit obligations at year end:
Discount rate	4.30%	4.30%
Weighted average assumptions for net periodic cost for the year:
Discount rate	4.30%	5.20%
Assumed health care cost trend rates:
Health care cost trend rate assumed for next year	7.60%	8.00%
Rate that the cost trend rate gradually declines to	5.00%	5.00%
Year that the rate reaches the rate it is assumed to remain at	2020	2020