Obligations under Guarantees and Other Off-balance Sheet Instruments (Maximum Potential Amount of Future Payments Classified Based upon Internal Credit Ratings) (Detail) - JPY (¥) ¥ in Billions	12 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	¥ 86,790	¥ 81,144
Standby letters of credit and financial guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	5,992	5,320
Performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	4,851	4,788
Standby letters of credit, financial and performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	10,843	10,108
Normal \| Standby letters of credit and financial guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	5,694	5,161
Normal \| Performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	4,734	4,685
Normal \| Standby letters of credit, financial and performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	¥ 10,428	9,846
Close Watch \| Minimum
Guarantor Obligations [Line Items]
Accruing loans contractually past due, in day	90 days
Close Watch \| Standby letters of credit and financial guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	¥ 282	123
Close Watch \| Performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	45	45
Close Watch \| Standby letters of credit, financial and performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	327	168
Likely to become Bankrupt or Legally/Virtually Bankrupt \| Standby letters of credit and financial guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	14	18
Likely to become Bankrupt or Legally/Virtually Bankrupt \| Performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	23	29
Likely to become Bankrupt or Legally/Virtually Bankrupt \| Standby letters of credit, financial and performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	37	47
Not rated \| Standby letters of credit and financial guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	2	18
Not rated \| Performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	49	29
Not rated \| Standby letters of credit, financial and performance guarantees
Guarantor Obligations [Line Items]
Maximum potential / Contractual or Notional amount, Amount by borrower grade	¥ 51	¥ 47