Contingent Liabilities and Guarantees - Guarantees (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	$ 15,116
Liability Recorded	200
Written Put Options [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	2,139
Liability Recorded	128
Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	11,426
Liability Recorded	64
Recourse Agreement with FNMA [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	1,547
Liability Recorded	4
Return Guarantee Agreement with LIHTC Investors [Member]
Guarantor Obligations [Line Items]
Maximum Potential Undiscounted Future Payments	4
Liability Recorded	$ 4