Fair value measurements - Geographic allocations of trading assets related to government, agency and municipal securities (Detail) - Government, agency and municipal securities [Member] - JPY (¥) ¥ in Billions		Mar. 31, 2017	Mar. 31, 2016
Fair value, concentration of credit risk [Line items]
Trading assets	[1]	¥ 6,335	¥ 8,248
Japan [Member]
Fair value, concentration of credit risk [Line items]
Trading assets		2,494	3,188
U.S. [Member]
Fair value, concentration of credit risk [Line items]
Trading assets		2,047	2,445
EU [Member]
Fair value, concentration of credit risk [Line items]
Trading assets		1,315	2,197
Other [Member]
Fair value, concentration of credit risk [Line items]
Trading assets		¥ 479	¥ 418

[1] Other than above, there were \577 billion and \544 billion of government, agency and municipal securities reported within Other assets-Non-trading debt securities in the consolidated balance sheets as of March 31, 2016 and 2017, respectively. These securities are primarily Japanese government, agency and municipal securities.