Mortgage Servicing Rights - Additional Information (Detail) (USD $)	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2011
Mortgage Servicing Rights [Line Items]
Assumed amortization rate of mortgage servicing rights	20.00%
Unpaid principal balance of mortgage loans serviced for others	$ 2,910,000,000	$ 3,130,000,000
Investor loss reimbursement	$ 4,530,000	$ 157,000	$ 0