Investment Securities (Amortized Cost, Gross Unrealized Gains (Losses) and Fair Value of Investment Securities) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	¥ 22,951,473	¥ 30,718,631
Available-for-sale debt securities, Gross unrealized gains	38,854	36,004
Available-for-sale debt securities, Gross unrealized losses	355,986	341,467
Available-for-sale debt securities	22,634,341	30,413,168
Held-to-maturity debt securities, Amortized cost	26,012,561	23,272,185
Held-to-maturity debt securities, Gross unrealized gains	28,463	15,277
Held-to-maturity debt securities, Gross unrealized losses	1,147,794	640,469
Held-to-maturity debt securities, fair value	24,893,230	22,646,993
Japanese national government and Japanese government agency bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	14,179,972	22,314,148
Available-for-sale debt securities, Gross unrealized gains	124	2,326
Available-for-sale debt securities, Gross unrealized losses	263,553	232,617
Available-for-sale debt securities	13,916,543	22,083,857
Held-to-maturity debt securities, Amortized cost	16,590,035	14,354,434
Held-to-maturity debt securities, Gross unrealized gains	0	4
Held-to-maturity debt securities, Gross unrealized losses	849,169	395,239
Held-to-maturity debt securities, fair value	15,740,866	13,959,199
Japanese prefectural and municipal bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	177,037	319,774
Available-for-sale debt securities, Gross unrealized gains	1	3
Available-for-sale debt securities, Gross unrealized losses	10,587	9,779
Available-for-sale debt securities	166,451	309,998
Held-to-maturity debt securities, Amortized cost	2,847,328	2,545,627
Held-to-maturity debt securities, Gross unrealized gains	0	0
Held-to-maturity debt securities, Gross unrealized losses	165,266	93,141
Held-to-maturity debt securities, fair value	2,682,062	2,452,486
Foreign government and official institution bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	4,646,116	4,149,418
Available-for-sale debt securities, Gross unrealized gains	14,579	11,679
Available-for-sale debt securities, Gross unrealized losses	61,469	82,628
Available-for-sale debt securities	4,599,226	4,078,469
Corporate bonds
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	989,473	905,194
Available-for-sale debt securities, Gross unrealized gains	9,756	10,213
Available-for-sale debt securities, Gross unrealized losses	17,969	11,524
Available-for-sale debt securities	981,260	903,883
Held-to-maturity debt securities, Amortized cost	243,772	214,949
Held-to-maturity debt securities, Gross unrealized gains	0	0
Held-to-maturity debt securities, Gross unrealized losses	6,061	3,602
Held-to-maturity debt securities, fair value	237,711	211,347
Residential mortgage-backed securities
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	1,015,934	1,121,879
Available-for-sale debt securities, Gross unrealized gains	1,197	583
Available-for-sale debt securities, Gross unrealized losses	109	211
Available-for-sale debt securities	1,017,022	1,122,251
Held-to-maturity debt securities, Amortized cost	4,488,121	4,690,277
Held-to-maturity debt securities, Gross unrealized gains	25,121	10,826
Held-to-maturity debt securities, Gross unrealized losses	124,695	148,066
Held-to-maturity debt securities, fair value	4,388,547	4,553,037
Asset-backed securities
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	1,590,991	1,405,859
Available-for-sale debt securities, Gross unrealized gains	9,585	7,491
Available-for-sale debt securities, Gross unrealized losses	254	1,199
Available-for-sale debt securities	1,600,322	1,412,151
Held-to-maturity debt securities, Amortized cost	1,843,305	1,466,898
Held-to-maturity debt securities, Gross unrealized gains	3,342	4,447
Held-to-maturity debt securities, Gross unrealized losses	2,603	421
Held-to-maturity debt securities, fair value	1,844,044	1,470,924
Other debt securities
Investment Securities [Line Items]
Available-for-sale debt securities, Amortized cost	351,950	502,359
Available-for-sale debt securities, Gross unrealized gains	3,612	3,709
Available-for-sale debt securities, Gross unrealized losses	2,045	3,509
Available-for-sale debt securities	¥ 353,517	¥ 502,559

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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