Financial risk management - Summary of Credit Risk Exposure By Risk Rating Grades (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Disclosure of credit risk exposure [line items]
Financial assets	¥ 3,020,544	¥ 22,435,594
Gross carrying amount [member] \| Trade And Other Receivables Contract Assets [Member]
Disclosure of credit risk exposure [line items]
Financial assets	1,947,158	2,075,559
Gross carrying amount [member] \| Debt securities [member]
Disclosure of credit risk exposure [line items]
Financial assets		11,266,558
Gross carrying amount [member] \| AAA [Member] \| Debt securities [member]
Disclosure of credit risk exposure [line items]
Financial assets		895,392
Gross carrying amount [member] \| AA [Member] \| Debt securities [member]
Disclosure of credit risk exposure [line items]
Financial assets		3,245,007
Gross carrying amount [member] \| A [Member] \| Debt securities [member]
Disclosure of credit risk exposure [line items]
Financial assets		7,052,650
Gross carrying amount [member] \| BBB [Member] \| Debt securities [member]
Disclosure of credit risk exposure [line items]
Financial assets		27,660
Gross carrying amount [member] \| Other [Member] \| Debt securities [member]
Disclosure of credit risk exposure [line items]
Financial assets		45,849
Not past due or due within 30 days [member] \| Gross carrying amount [member] \| Trade And Other Receivables Contract Assets [Member]
Disclosure of credit risk exposure [line items]
Financial assets	1,831,302	1,972,150
Due over 30 to 90 days [member] \| Gross carrying amount [member] \| Trade And Other Receivables Contract Assets [Member]
Disclosure of credit risk exposure [line items]
Financial assets	66,618	46,897
Due over 90 days [member] \| Gross carrying amount [member] \| Trade And Other Receivables Contract Assets [Member]
Disclosure of credit risk exposure [line items]
Financial assets	¥ 49,238	¥ 56,512