Employee Benefits - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended	12 Months Ended		3 Months Ended		3 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Jun. 30, 2013	Jun. 30, 2013 Pension Remeasurement Credit	Dec. 31, 2012 Pension Remeasurement Credit	Jun. 30, 2013 Pension Remeasurement Gain Primarily Driven By Increase In Discount Rate Assumption	Jun. 30, 2013 Pension Charges Driven By Difference Between Expected Return on Assets and Annualized Actual Return On Assets And Other	Sep. 30, 2013 Other Postretirement Benefit Plans	Sep. 30, 2013 Other Postretirement Benefit Plans	Sep. 30, 2013 Non Qualified Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Pension remeasurement credit (charges)		$ 237,000,000			$ 200,000,000		$ 300,000,000	$ (100,000,000)
Discount rate basis point increase (decrease)					75
Discount rate				5.00%		4.20%
Expected return on plan assets			7.50%
Actual return on assets					7.20%
Amount paid in severance benefits over the period	100,000,000	300,000,000
Post employment benefits liability	700,000,000	700,000,000
Defined benefit plan contributions by employer									$ 400,000,000	$ 1,100,000,000	$ 100,000,000