Collateralized transactions - Assets Owned, Pledged as Collateral (Detail) (JPY ¥) In Millions, unless otherwise specified	Mar. 31, 2013		Mar. 31, 2012
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	¥ 3,374,749		¥ 3,171,738
Non-trading debt securities	49,811		54,969
Investments in and advances to affiliated companies	37,636		33,921
Equities and convertible securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	86,108		47,966
Government and government agency securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	1,314,277		1,333,482
Bank and corporate debt securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	161,233		139,863
Commercial mortgage-backed securities ("CMBS") [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	33,723		40,183
Residential mortgage-backed securities ("RMBS") [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	1,674,898		1,527,946
Collateralized debt obligations ("CDO") and other [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	84,065	[1]	82,298	[1]
Investment trust funds and other [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	16,335
Deposits with stock exchanges and other segregated cash [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Trading assets (without right to sell or repledge)	¥ 4,110

[1] Includes CLO and ABS those on credit card loans, auto loans and student loans.