Employee Benefits - Weighted-Average Rates to Determine Net Postretirement Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Discount rate	4.00%	4.75%	5.25%
Expected return on plan assets	7.25%	7.75%	8.25%
Other Postretirement Benefit Plans, Defined Benefit [Member]
Discount rate	4.00%	4.75%	5.25%
Expected return on plan assets	5.58%	5.45%	5.46%