Derivative instruments and hedging activities - Written Credit Derivatives and Purchased Credit Protection (Detail) - JPY (¥) ¥ in Billions		12 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ 195	¥ (51)
Maximum potential payout / Notional		21,828	25,825
Notional Purchased credit protection		17,367	22,307
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	131	(21)
Maximum potential payout / Notional		15,609	18,808
Notional Purchased credit protection		12,796	16,519
Credit default indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	52	(22)
Maximum potential payout / Notional		5,797	6,044
Notional Purchased credit protection		4,295	5,240
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	12	(8)
Maximum potential payout / Notional		355	673
Notional Purchased credit protection		209	293
Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	0	0
Maximum potential payout / Notional		67	300
Notional Purchased credit protection		67	255
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,647	5,389
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,658	4,146
Less than 1 year [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		918	919
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 71	¥ 324
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		¥ 7,163	¥ 9,539
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,292	7,396
1 to 3 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,623	1,926
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 248	¥ 217
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		¥ 7,848	¥ 8,491
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,252	5,657
3 to 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,505	2,462
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		24	117
3 to 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		67	255
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,170	2,406
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,407	1,609
More than 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		751	737
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 12	15
More than 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			¥ 45

[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.