Summary of Subsequent Defaults (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Mar. 31, 2012
Financing Receivable, Modifications
Number of contracts	574	[1]	687	[1]
Recorded Investment in Loans Modified in a TDR During the Period	$ 12	[1]	$ 21	[1]
Commercial Portfolio Segment | Commercial Mortgage Loans, Non Owner Occupied
Financing Receivable, Modifications
Number of contracts			1	[1]
Recorded Investment in Loans Modified in a TDR During the Period			1	[1]
Commercial Portfolio Segment | Commercial Construction
Financing Receivable, Modifications
Number of contracts			2	[1]
Recorded Investment in Loans Modified in a TDR During the Period			3	[1]
Consumer Portfolio Segment | Home Equity
Financing Receivable, Modifications
Number of contracts	14	[1]	31	[1]
Recorded Investment in Loans Modified in a TDR During the Period	1	[1]	2	[1]
Consumer Portfolio Segment | Automobile Loans
Financing Receivable, Modifications
Number of contracts	3	[1]	12	[1]
Consumer Portfolio Segment | Credit Card
Financing Receivable, Modifications
Number of contracts	515	[1]	577	[1]
Recorded Investment in Loans Modified in a TDR During the Period	3
Residential Mortgage Loans
Financing Receivable, Modifications
Number of contracts	42	[1]	64	[1]
Recorded Investment in Loans Modified in a TDR During the Period	$ 8	[1]	$ 11	[1]

[1]  Excludes all loans and leases held for sale and loans acquired with deteriorated credit quality.