Lease Transactions (Narrative) (Detail) - JPY (¥) ¥ in Millions	12 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Leases [Abstract]
Right-of-use assets of finance leases, in consolidated balance sheets line item	Premises and equipment—net (Notes 5 and 7)	Premises and equipment—net (Notes 5 and 7)
Right-of-use assets of finance leases	¥ 24,667	¥ 16,402
Lease liabilities of finance leases, in consolidated balance sheets line item	Long-term debt (including ¥431,338 and ¥234,909 measured at fair value under the fair value option in 2023 and 2024) (Notes 7, 9, 12 and 31)	Long-term debt (including ¥431,338 and ¥234,909 measured at fair value under the fair value option in 2023 and 2024) (Notes 7, 9, 12 and 31)
Lease liabilities of finance leases	¥ 26,371	¥ 19,470
Right-of-use assets of operating leases, in consolidated balance sheets line item	Other Assets	Other Assets
Right-of-use assets of operating leases	¥ 234,386	¥ 227,112
Lease liabilities of operating leases, in consolidated balance sheets line item	Other Liabilities	Other Liabilities
Amount on balance sheet	¥ 328,559	¥ 337,391
Operating Lease, Impairment Loss	¥ 18,112	¥ 15,720

Present value of lessee's discounted obligation for lease payments from finance lease.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147478964/842-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479041/842-20-45-1

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
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-Section 45
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-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479041/842-20-45-2

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Amount, after accumulated amortization, of right-of-use asset from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
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Amount of loss from impairment of right-of-use asset from operating lease.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 25
-Paragraph 6
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479365/842-20-25-6

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Present value of lessee's discounted obligation for lease payments from operating lease.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amount of lessee's right to use underlying asset under operating lease.

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