Employee Postretirement Benefits - Schedule of Assumptions Used to Determine Benefits (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.196%	4.797%
Rate of compensation increase - weighted	3.76%	3.83%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.18%	4.744%
Rate of compensation increase - weighted	3.74%	3.82%
Healthcare cost trend rate
Immediate rate	7.50%	7.75%
Ultimate rate	4.50%	4.50%
Year rate reaches ultimate	2025	2025
Net periodic benefit cost [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.797%	5.30%	5.75%
Expected long-term return on plan assets	7.50%	7.75%	8.25%
Rate of compensation increase	3.83%	3.88%	4.25%
Net periodic benefit cost [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.744%	5.25%	5.60%
Rate of compensation increase	3.82%	3.87%	4.25%
Healthcare cost trend rate
Immediate rate	7.75%	8.00%	8.00%
Ultimate rate	4.50%	4.50%	5.00%
Year rate reaches ultimate	2025	2023	2017