Charges and Credits - Summary of Merger and Integration Related Charges (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Jun. 30, 2013	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Sep. 30, 2013	Sep. 30, 2012
Restructuring Cost And Reserve [Line Items]
Charges, Pretax		$ 32				$ 68
Charges, Tax	19				19
Charges, Net	(683)	28	21	13	(591)	62
Merger - Related Integration Costs
Restructuring Cost And Reserve [Line Items]
Charges, Pretax		32	22	14		68
Merger-related transaction costs
Restructuring Cost And Reserve [Line Items]
Charges, Tax		$ 4	$ 1	$ 1		$ 6