Collateralized transactions - Assets owned, pledged as collateral ,primarily to stock exchanges and clearing organizations (Detail) - JPY (¥) ¥ in Millions		Mar. 31, 2019	Mar. 31, 2018
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		¥ 3,774,680	¥ 3,834,905
Non-trading debt securities		1,031	23,566
Investments in and advances to affiliated companies		501	12,042
Equities and convertible securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		135,927	156,429
Government and government agency securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		984,429	956,089
Bank and corporate debt securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		61,547	65,864
Commercial mortgage-backed securities ("CMBS") [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		0	1
Residential mortgage-backed securities ("RMBS") [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		2,535,244	2,618,336
Collateralized debt obligations ("CDOs") and other [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets	[1]	42,607	30,497
Investment trust funds and other [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets		¥ 14,926	¥ 7,689

[1] Includes CLOs and ABS such as those secured on credit card loans, auto loans and student loans.