Commitments, contingencies and guarantees - Maturity Information on Derivative Contracts and Standby Letters of Credit and Other Guarantees (Detail) (JPY ¥) In Millions, unless otherwise specified	Mar. 31, 2013		Mar. 31, 2012
Maximum potential payout/ Notional, Years to Maturity, Less than 1 year [Member]
Commitments And Contingencies Disclosure [Line Items]
Maximum potential payout/Notional total, Derivative contracts	¥ 8
Maximum potential payout/Notional total, Standby letters of credit and other guarantees	39,561,164
Maximum potential payout/ Notional, Years to Maturity, 1 to 3 years [Member]
Commitments And Contingencies Disclosure [Line Items]
Maximum potential payout/Notional total, Derivative contracts	319
Maximum potential payout/Notional total, Standby letters of credit and other guarantees	31,284,453
Maximum potential payout/ Notional, Years to Maturity, 3 to 5 years [Member]
Commitments And Contingencies Disclosure [Line Items]
Maximum potential payout/Notional total, Derivative contracts	668
Maximum potential payout/Notional total, Standby letters of credit and other guarantees	13,333,026
Maximum potential payout/ Notional, Years to Maturity, More than 5 years [Member]
Commitments And Contingencies Disclosure [Line Items]
Maximum potential payout/Notional total, Derivative contracts	8,089
Maximum potential payout/Notional total, Standby letters of credit and other guarantees	39,801,838
Derivative contracts [Member]
Commitments And Contingencies Disclosure [Line Items]
Carrying value, Derivative contracts	4,510,650	[1],[2]	3,997,315	[1],[2]
Maximum potential payout/Notional total, Derivative contracts	123,980,481	[1],[2]	107,572,427	[1],[2]
Standby letters of credit and other guarantees [Member]
Commitments And Contingencies Disclosure [Line Items]
Carrying value, Standby letters of credit and other guarantees	277	[3]	264	[3]
Maximum potential payout/Notional total, Standby letters of credit and other guarantees	¥ 9,084	[3]	¥ 21,674	[3]

[1] Credit derivatives are disclosed in Note 3 "Derivative instruments and hedging activities" and are excluded from derivative contracts.

[2] Derivative contracts primarily consist of equity, interest rate and foreign exchange contracts.

[3] Collateral held in connection with standby letters of credit and other guarantees as of March 31, 2012 was \6,377 million and as of March 31, 2013 is \6,374 million.