Fair value measurements - Schedule of unrealized gains and losses recognized for Level 3 financial instruments (Detail) - Recurring [Member] - Level 3 [Member] - JPY (¥) ¥ in Billions		3 Months Ended				6 Months Ended
		Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	¥ (4)		¥ 4		¥ (31)		¥ 7
Unrealized gains/(losses), Liabilities	[1]	(6)		6		(20)		8
Trading assets and private equity and debt investments [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	2		(5)		(2)		(8)
Trading assets and private equity and debt investments [Member] | Equities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		0		1		0
Trading assets and private equity and debt investments [Member] | Private equity and debt investments [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	1		1		2		2
Trading assets and private equity and debt investments [Member] | Japanese agency and municipal securities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		0		0		0
Trading assets and private equity and debt investments [Member] | Foreign government, agency and municipal securities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		0		0		0
Trading assets and private equity and debt investments [Member] | Bank and corporate debt securities and loans for trading purposes [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	1		(3)		0		(6)
Trading assets and private equity and debt investments [Member] | Commercial mortgage-backed securities ("CMBS") [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		0		0		0
Trading assets and private equity and debt investments [Member] | Residential mortgage-backed securities ("RMBS") [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		(1)		0		(1)
Trading assets and private equity and debt investments [Member] | Real estate-backed securities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		0		(2)		0
Trading assets and private equity and debt investments [Member] | Collateralized debt obligations ("CDOs") and other [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		(2)		(3)		(3)
Trading assets and private equity and debt investments [Member] | Investment trust funds and other [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0		0				0
Equity contracts [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1],[2]	(12)		5		(38)		5
Interest rate contracts [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1],[2]	2		(4)		(10)		(5)
Credit contracts [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1],[2]	(4)		0		12		(1)
Foreign exchange contracts [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1],[2]	0		3		(15)		2
Commodity contracts [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1],[2]			0				0
Derivatives, net [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1],[2]	(14)		4		(51)		1
Subtotal [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	(12)		(1)		(53)		(7)
Loans and receivables [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	2		0		1		1
Collateralized agreements [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	0				0		0
Other [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Assets	[1]	6		5		21		13
Trading liabilities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities	[1]	0		0		0		0
Trading liabilities [Member] | Equities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities	[1]			0		0		0
Trading liabilities [Member] | Foreign government, agency and municipal securities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities	[1]	0		0		0		0
Trading liabilities [Member] | Bank and corporate debt securities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities	[1]	0		0		0		0
Trading liabilities [Member] | Collateralized debt obligations ("CDOs") and other [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities		0	[1]		[1]	0	[1]
Short-term borrowings [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities		1	[1],[3]	1		2	[1],[3]	1	[1],[3]
Payables and deposits [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities		0	[1]	0		0	[1],[3]	0	[1],[3]
Long-term borrowings [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities		(3)	[1],[3]	5		(15)	[1],[3]	7	[1],[3]
Other liabilities [Member]
Fair value, assets and liabilities measured on recurring basis, unobservable input reconciliation [Line items]
Unrealized gains/(losses), Liabilities	[1]	¥ (4)		¥ 0		¥ (7)		¥ 0

[1] Includes gains and losses reported within Net gain on trading, Gain (loss) on private equity and debt investments, and also within Gain on investments in equity securities, Revenue—Other and Non-interest expenses—Other, Interest and dividends and Interest expense in the consolidated statements of income.

[2] Each derivative classification includes derivatives with multiple risk underlyings. For example, interest rate contracts include complex derivatives referencing interest rate risk as well as foreign exchange risk or other factors such as prepayment rates. Credit contracts include credit default swaps as well as derivatives referencing corporate and government debt securities.

[3] Includes changes in unrealized gains and losses in Other comprehensive income (loss) for recurring Level 3 fair value measurements held at the end of the reporting period. They were ¥2 billion and ¥2 billion for the six months ended September 30, 2019 and 2020, respectively and ¥1 billion and ¥(2) billion for the three months ended September 30, 2019 and 2020, respectively.