Fair value measurements - Gains (losses) due to changes in fair value for financial instruments measured at fair value using fair value option (Detail) - JPY (¥) ¥ in Billions		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Short-term borrowings [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[2]	¥ (42)	¥ 14	¥ 11
Collateralized financing [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[3]	(1)	6	(2)
Long-term borrowings [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[2],[4]	7	79	(7)
Other liabilities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[5]	0	(2)	0
Total
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1]	(36)	97	2
Trading assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[6]	1	1	0
Private equity investments [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[6]	0	1	1
Loans and receivables [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1]	1	5	4
Collateralized agreements [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[3]	6	2	4
Other assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1],[6]	10	(22)	6
Total
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains/(Losses)	[1]	¥ 18	¥ (13)	¥ 15

[1] Includes gains and losses reported primarily within Net gain on trading, Gain on private equity investments and Revenue-Other in the consolidated statements of income.

[2] Includes structured notes and other financial liabilities.

[3] Includes reverse repurchase and repurchase agreements.

[4] Includes secured financing transactions arising from transfers of financial assets which did not meet the criteria for sales accounting.

[5] Includes unfunded written loan commitments.

[6] Includes equity investments that would have been accounted for under the equity method had Nomura not chosen to elect the fair value option.