Restructuring initiatives - Additional information (Detail) - JPY (¥) ¥ in Millions	6 Months Ended
Sep. 30, 2019	Mar. 31, 2020
Restructuring Initiatives [Abstract]
Liabilities relating to restructuring costs including currency translation adjustments		¥ 507
Amount paid as severance costs		9,305
Branch consolidation costs	¥ 4,231
Liabilities relating to branch consolidation costs		¥ 813

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Includes, but is not limited to, one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and termination benefits associated with an ongoing benefit arrangement. Excludes expenses associated with special or contractual termination benefits, a discontinued operation or an asset retirement obligation.

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