Employee Benefits - Weighted Average Assumptions Used to Determine Postretirement Benefit Expense and Obligation (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Expense [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.60%	5.50%
Current medical cost trend rate	7.00%	8.00%
Ultimate medical cost trend rate	4.75%	4.75%
Medical cost trend rate decreases to ultimate rate in year	2021	2016
Liability [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.00%	4.60%
Current medical cost trend rate	6.75%	7.00%
Ultimate medical cost trend rate	4.75%	4.75%
Medical cost trend rate decreases to ultimate rate in year	2021	2021