Derivative instruments and hedging activities - Written Credit Derivatives and Purchased Credit Protection (Detail) - JPY (¥) ¥ in Billions		12 Months Ended
		Mar. 31, 2015	Mar. 31, 2014
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ (51)	¥ (145)
Maximum potential payout / Notional		25,825	32,402
Notional Purchased credit protection		22,307	28,038
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(21)	(235)
Maximum potential payout / Notional		18,808	21,070
Notional Purchased credit protection		16,519	18,689
Credit default indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(22)	(32)
Maximum potential payout / Notional		6,044	9,082
Notional Purchased credit protection		5,240	7,704
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(8)	123
Maximum potential payout / Notional		673	1,574
Notional Purchased credit protection		293	1,097
Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	0	(1)
Maximum potential payout / Notional		300	676
Notional Purchased credit protection		255	548
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,389	5,905
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,146	4,167
Less than 1 year [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		919	1,215
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 324	¥ 523
Less than 1 year [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
1 to 3 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 9,539	¥ 12,256
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		7,396	8,306
1 to 3 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,926	3,552
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 217	¥ 398
1 to 3 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
3 to 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 8,491	¥ 10,897
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,657	6,610
3 to 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,462	3,582
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		117	201
3 to 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		255	504
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,406	3,344
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,609	1,987
More than 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		737	733
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		15	452
More than 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 45	¥ 172

[1] Carrying value amounts are shown on a gross basis prior to cash collateral or counterparty netting. Asset balances represent positive fair value amounts caused by tightening of credit spreads of underlyings since inception of the credit derivative contracts.