Investment Securities (Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Investment Securities) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2025	Mar. 31, 2024
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	¥ 30,718,631	¥ 31,668,629
Available-for-sale debt securities, Gross unrealized gains	36,004	34,586
Available-for-sale debt securities, Gross unrealized losses	341,467	280,248
Available-for-sale debt securities	30,413,168	31,422,967
Held-to-maturity debt securities	23,272,185	24,843,962
Held-to-maturity debt securities, Gross unrealized gains	15,277	13,720
Held-to-maturity debt securities, Gross unrealized losses	640,469	300,168
Held-to-maturity debt securities, fair value	22,646,993	24,557,514
Japanese national government and Japanese government agency bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	22,314,148	23,287,730
Available-for-sale debt securities, Gross unrealized gains	2,326	5,768
Available-for-sale debt securities, Gross unrealized losses	232,617	126,098
Available-for-sale debt securities	22,083,857	23,167,400
Held-to-maturity debt securities	14,354,434	15,191,741
Held-to-maturity debt securities, Gross unrealized gains	4	4,745
Held-to-maturity debt securities, Gross unrealized losses	395,239	97,603
Held-to-maturity debt securities, fair value	13,959,199	15,098,883
Japanese prefectural and municipal bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	319,774	1,055,564
Available-for-sale debt securities, Gross unrealized gains	3	227
Available-for-sale debt securities, Gross unrealized losses	9,779	9,800
Available-for-sale debt securities	309,998	1,045,991
Held-to-maturity debt securities	2,545,627	1,999,182
Held-to-maturity debt securities, Gross unrealized gains	0	2,048
Held-to-maturity debt securities, Gross unrealized losses	93,141	16,328
Held-to-maturity debt securities, fair value	2,452,486	1,984,902
Foreign government and official institution bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	4,149,418	3,428,199
Available-for-sale debt securities, Gross unrealized gains	11,679	4,981
Available-for-sale debt securities, Gross unrealized losses	82,628	130,629
Available-for-sale debt securities	4,078,469	3,302,551
Corporate bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	905,194	1,019,832
Available-for-sale debt securities, Gross unrealized gains	10,213	10,685
Available-for-sale debt securities, Gross unrealized losses	11,524	4,049
Available-for-sale debt securities	903,883	1,026,468
Held-to-maturity debt securities	214,949	119,490
Held-to-maturity debt securities, Gross unrealized gains	0	93
Held-to-maturity debt securities, Gross unrealized losses	3,602	179
Held-to-maturity debt securities, fair value	211,347	119,404
Residential mortgage-backed securities
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	1,121,879	1,229,615
Available-for-sale debt securities, Gross unrealized gains	583	255
Available-for-sale debt securities, Gross unrealized losses	211	345
Available-for-sale debt securities	1,122,251	1,229,525
Held-to-maturity debt securities	4,690,277	4,952,083
Held-to-maturity debt securities, Gross unrealized gains	10,826	3,153
Held-to-maturity debt securities, Gross unrealized losses	148,066	179,537
Held-to-maturity debt securities, fair value	4,553,037	4,775,699
Asset-backed securities
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	1,405,859	1,239,772
Available-for-sale debt securities, Gross unrealized gains	7,491	9,251
Available-for-sale debt securities, Gross unrealized losses	1,199	1,877
Available-for-sale debt securities	1,412,151	1,247,146
Held-to-maturity debt securities	1,466,898	2,581,466
Held-to-maturity debt securities, Gross unrealized gains	4,447	3,681
Held-to-maturity debt securities, Gross unrealized losses	421	6,521
Held-to-maturity debt securities, fair value	1,470,924	2,578,626
Other debt securities
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	502,359	407,917
Available-for-sale debt securities, Gross unrealized gains	3,709	3,419
Available-for-sale debt securities, Gross unrealized losses	3,509	7,450
Available-for-sale debt securities	¥ 502,559	¥ 403,886

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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