Benefit Obligations - Actuarial Assumptions (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate to determine net periodic cost	4.30%	5.25%	5.75%
Discount rate to determine the benefit obligation as of December 31	3.80%	4.30%	5.25%
Expected average rate of return on plan assets	7.75%	7.75%	8.00%
Post-retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate to determine net periodic cost	4.50%	5.70%	6.00%
Discount rate to determine the benefit obligation as of December 31	4.20%	4.50%	5.70%
Rate of compensation increase to determine net periodic cost	3.50%	3.50%	4.00%
Rate of compensation increase to determine the benefit obligation as of December 31	3.50%	3.50%	3.50%
Expected average rate of return on plan assets	5.70%	6.20%	7.00%