Employee benefits - Additional Information (Detail) - JPY (¥) ¥ in Billions	12 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Disclosure of defined benefit plans [line items]
Percentage of indemnities cover under defined benefits plan	65.00%
Defined Benefit Plan Equity Securities [Member] \| Japanese Defined Benefit Plans [Member]
Disclosure of defined benefit plans [line items]
Defined benefit plan percentage of amount contributed to fair value of plan assets	14.00%	18.00%
Defined Benefit Plan Equity Securities [Member] \| Foreign Defined Benefit Plan [Member]
Disclosure of defined benefit plans [line items]
Defined benefit plan percentage of amount contributed to fair value of plan assets	2.00%	2.00%
Defined Benefit Plan Fixed Income Securities [Member] \| Japanese Defined Benefit Plans [Member]
Disclosure of defined benefit plans [line items]
Defined benefit plan percentage of amount contributed to fair value of plan assets	57.00%	51.00%
Defined Benefit Plan Fixed Income Securities [Member] \| Foreign Defined Benefit Plan [Member]
Disclosure of defined benefit plans [line items]
Defined benefit plan percentage of amount contributed to fair value of plan assets	10.00%	13.00%
Defined Benefit Plan Other Investments [Member] \| Japanese Defined Benefit Plans [Member]
Disclosure of defined benefit plans [line items]
Defined benefit plan percentage of amount contributed to fair value of plan assets	29.00%	31.00%
Defined Benefit Plan Other Investments [Member] \| Foreign Defined Benefit Plan [Member]
Disclosure of defined benefit plans [line items]
Defined benefit plan percentage of amount contributed to fair value of plan assets	88.00%	85.00%
Defined Benefit Plan Securities [Member] \| Plan assets [member] \| Japanese Defined Benefit Plans [Member]
Disclosure of defined benefit plans [line items]
Expected contributions to plan assets by employer during the next fiscal year	¥ 2
Defined Benefit Plan Securities [Member] \| Plan assets [member] \| Foreign Defined Benefit Plan [Member]
Disclosure of defined benefit plans [line items]
Expected contributions to plan assets by employer during the next fiscal year	¥ 6