v3.26.1
Fair Value (Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis, Including Fair Value Option Elected) (Detail) - JPY (¥)
¥ in Millions
Mar. 31, 2026
Mar. 31, 2025
Assets:    
Securities measured at net asset value [1],[2]   ¥ 103,000
Available-for-sale securities ¥ 21,848,687 15,058,617
Equity securities measured at net asset value [1]   339,000
Liabilities:    
Other short-term borrowings 106,746 244,157
Long-term debt 4,693,080 3,764,171
Fair Value, Measurements, Recurring    
Assets:    
Securities measured at net asset value [1],[2] 108,000  
Equity securities with readily determinable fair values 4,545,000 3,578,000
Equity securities measured at net asset value [1] 479,000  
Other investments 90,000 88,000
Total assets measured at fair value 75,737,000 56,816,000
Liabilities:    
Trading securities sold, not yet purchased 4,597,000 4,941,000
Other short-term borrowings [3] 107,000 244,000
Long-term debt [3] 4,693,000 3,764,000
Other liabilities 77,000 125,000
Total liabilities measured at fair value 33,679,000 25,341,000
Fair Value, Measurements, Recurring | Interest rate contracts    
Assets:    
Assets, derivatives 16,440,000 10,830,000
Liabilities:    
Liabilities, derivatives 16,892,000 10,910,000
Fair Value, Measurements, Recurring | Foreign exchange contracts    
Assets:    
Assets, derivatives 6,725,000 4,534,000
Liabilities:    
Liabilities, derivatives 6,566,000 4,813,000
Fair Value, Measurements, Recurring | Equity-related contracts    
Assets:    
Assets, derivatives 355,000 261,000
Liabilities:    
Liabilities, derivatives 550,000 313,000
Fair Value, Measurements, Recurring | Credit-related contracts    
Assets:    
Assets, derivatives 206,000 245,000
Liabilities:    
Liabilities, derivatives 134,000 210,000
Fair Value, Measurements, Recurring | Other contracts    
Assets:    
Assets, derivatives 57,000 21,000
Liabilities:    
Liabilities, derivatives 63,000 21,000
Fair Value, Measurements, Recurring | Japanese government bonds    
Assets:    
Assets, trading securities [2] 3,134,000 2,126,000
Available-for-sale securities 14,925,000 8,362,000
Fair Value, Measurements, Recurring | Japanese local government bonds    
Assets:    
Assets, trading securities [2] 122,000 98,000
Available-for-sale securities 150,000 548,000
Fair Value, Measurements, Recurring | U.S. Treasury bonds and federal agency securities    
Assets:    
Assets, trading securities [2] 6,447,000 5,308,000
Available-for-sale securities 138,000 128,000
Fair Value, Measurements, Recurring | Other foreign government bonds    
Assets:    
Assets, trading securities [2] 2,049,000 1,703,000
Available-for-sale securities 3,564,000 2,591,000
Fair Value, Measurements, Recurring | Agency mortgage-backed securities    
Assets:    
Assets, trading securities [2] 4,695,000 5,494,000
Available-for-sale securities 282,000 316,000
Fair Value, Measurements, Recurring | Certificates of deposit and commercial paper    
Assets:    
Assets, trading securities [2] 1,103,000 956,000
Fair Value, Measurements, Recurring | Corporate bonds and other    
Assets:    
Assets, trading securities [2],[4] 3,384,000 3,030,000
Fair Value, Measurements, Recurring | Equity securities    
Assets:    
Assets, trading securities [2] 3,873,000 2,888,000
Fair Value, Measurements, Recurring | Residential mortgage-backed securities    
Assets:    
Available-for-sale securities 12,000 21,000
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities    
Assets:    
Available-for-sale securities 914,000 854,000
Fair Value, Measurements, Recurring | Japanese corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 923,000 1,438,000
Fair Value, Measurements, Recurring | Foreign corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 942,000 801,000
Fair Value, Measurements, Recurring | Other Assets    
Assets:    
Other assets 77,000 154,000 [5]
Fair Value, Measurements, Recurring | Level 1    
Assets:    
Equity securities with readily determinable fair values 3,459,000 3,034,000
Other investments 1,000 2,000
Total assets measured at fair value 32,425,000 22,499,000
Liabilities:    
Trading securities sold, not yet purchased 3,297,000 3,320,000
Other short-term borrowings [3] 0  
Long-term debt [3] 0  
Other liabilities 2,000 5,000
Total liabilities measured at fair value 3,546,000 3,424,000
Fair Value, Measurements, Recurring | Level 1 | Interest rate contracts    
Assets:    
Assets, derivatives 38,000 25,000
Liabilities:    
Liabilities, derivatives 39,000 18,000
Fair Value, Measurements, Recurring | Level 1 | Foreign exchange contracts    
Assets:    
Assets, derivatives 0  
Liabilities:    
Liabilities, derivatives 0  
Fair Value, Measurements, Recurring | Level 1 | Equity-related contracts    
Assets:    
Assets, derivatives 130,000 83,000
Liabilities:    
Liabilities, derivatives 185,000 71,000
Fair Value, Measurements, Recurring | Level 1 | Credit-related contracts    
Assets:    
Assets, derivatives 0  
Liabilities:    
Liabilities, derivatives 0  
Fair Value, Measurements, Recurring | Level 1 | Other contracts    
Assets:    
Assets, derivatives 31,000 6,000
Liabilities:    
Liabilities, derivatives 22,000 9,000
Fair Value, Measurements, Recurring | Level 1 | Japanese government bonds    
Assets:    
Assets, trading securities [2] 3,106,000 2,111,000
Available-for-sale securities 14,205,000 7,938,000
Fair Value, Measurements, Recurring | Level 1 | Japanese local government bonds    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury bonds and federal agency securities    
Assets:    
Assets, trading securities [2] 4,658,000 4,274,000
Available-for-sale securities 138,000 128,000
Fair Value, Measurements, Recurring | Level 1 | Other foreign government bonds    
Assets:    
Assets, trading securities [2] 1,159,000 975,000
Available-for-sale securities 1,804,000 1,150,000
Fair Value, Measurements, Recurring | Level 1 | Agency mortgage-backed securities    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit and commercial paper    
Assets:    
Assets, trading securities [2] 0  
Fair Value, Measurements, Recurring | Level 1 | Corporate bonds and other    
Assets:    
Assets, trading securities [2],[4] 1,000 6,000
Fair Value, Measurements, Recurring | Level 1 | Equity securities    
Assets:    
Assets, trading securities [2] 3,694,000 2,735,000
Fair Value, Measurements, Recurring | Level 1 | Residential mortgage-backed securities    
Assets:    
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 1 | Commercial mortgage-backed securities    
Assets:    
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 1 | Japanese corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 1 | Foreign corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 1 | Other Assets    
Assets:    
Other assets 2,000 34,000 [5]
Fair Value, Measurements, Recurring | Level 2    
Assets:    
Equity securities with readily determinable fair values 1,086,000 545,000
Other investments 0  
Total assets measured at fair value 42,115,000 33,325,000
Liabilities:    
Trading securities sold, not yet purchased 1,300,000 1,621,000
Other short-term borrowings [3] 107,000 244,000
Long-term debt [3] 4,124,000 3,276,000
Other liabilities 36,000 82,000
Total liabilities measured at fair value 29,325,000 21,298,000
Fair Value, Measurements, Recurring | Level 2 | Interest rate contracts    
Assets:    
Assets, derivatives 16,390,000 10,801,000
Liabilities:    
Liabilities, derivatives 16,838,000 10,885,000
Fair Value, Measurements, Recurring | Level 2 | Foreign exchange contracts    
Assets:    
Assets, derivatives 6,719,000 4,526,000
Liabilities:    
Liabilities, derivatives 6,563,000 4,811,000
Fair Value, Measurements, Recurring | Level 2 | Equity-related contracts    
Assets:    
Assets, derivatives 210,000 175,000
Liabilities:    
Liabilities, derivatives 194,000 165,000
Fair Value, Measurements, Recurring | Level 2 | Credit-related contracts    
Assets:    
Assets, derivatives 202,000 242,000
Liabilities:    
Liabilities, derivatives 131,000 205,000
Fair Value, Measurements, Recurring | Level 2 | Other contracts    
Assets:    
Assets, derivatives 18,000 10,000
Liabilities:    
Liabilities, derivatives 33,000 9,000
Fair Value, Measurements, Recurring | Level 2 | Japanese government bonds    
Assets:    
Assets, trading securities [2] 27,000 16,000
Available-for-sale securities 720,000 424,000
Fair Value, Measurements, Recurring | Level 2 | Japanese local government bonds    
Assets:    
Assets, trading securities [2] 122,000 98,000
Available-for-sale securities 150,000 548,000
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury bonds and federal agency securities    
Assets:    
Assets, trading securities [2] 1,789,000 1,034,000
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 2 | Other foreign government bonds    
Assets:    
Assets, trading securities [2] 890,000 728,000
Available-for-sale securities 1,760,000 1,441,000
Fair Value, Measurements, Recurring | Level 2 | Agency mortgage-backed securities    
Assets:    
Assets, trading securities [2] 4,695,000 5,494,000
Available-for-sale securities 282,000 316,000
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit and commercial paper    
Assets:    
Assets, trading securities [2] 1,103,000 956,000
Fair Value, Measurements, Recurring | Level 2 | Corporate bonds and other    
Assets:    
Assets, trading securities [2],[4] 3,207,000 2,851,000
Fair Value, Measurements, Recurring | Level 2 | Equity securities    
Assets:    
Assets, trading securities [2] 159,000 133,000
Fair Value, Measurements, Recurring | Level 2 | Residential mortgage-backed securities    
Assets:    
Available-for-sale securities 0 15,000
Fair Value, Measurements, Recurring | Level 2 | Commercial mortgage-backed securities    
Assets:    
Available-for-sale securities 914,000 851,000
Fair Value, Measurements, Recurring | Level 2 | Japanese corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 694,000 1,252,000
Fair Value, Measurements, Recurring | Level 2 | Foreign corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 942,000 788,000
Fair Value, Measurements, Recurring | Level 2 | Other Assets    
Assets:    
Other assets 36,000 82,000 [5]
Fair Value, Measurements, Recurring | Level 3    
Assets:    
Equity securities with readily determinable fair values 0  
Other investments 89,000 86,000
Total assets measured at fair value 610,000 550,000
Liabilities:    
Trading securities sold, not yet purchased 0  
Other short-term borrowings [3] 0  
Long-term debt [3] 569,000 489,000
Other liabilities 40,000 37,000
Total liabilities measured at fair value 808,000 619,000
Fair Value, Measurements, Recurring | Level 3 | Interest rate contracts    
Assets:    
Assets, derivatives 11,000 4,000
Liabilities:    
Liabilities, derivatives 15,000 6,000
Fair Value, Measurements, Recurring | Level 3 | Foreign exchange contracts    
Assets:    
Assets, derivatives 6,000 8,000
Liabilities:    
Liabilities, derivatives 3,000 2,000
Fair Value, Measurements, Recurring | Level 3 | Equity-related contracts    
Assets:    
Assets, derivatives 14,000 4,000
Liabilities:    
Liabilities, derivatives 171,000 77,000
Fair Value, Measurements, Recurring | Level 3 | Credit-related contracts    
Assets:    
Assets, derivatives 4,000 3,000
Liabilities:    
Liabilities, derivatives 3,000 5,000
Fair Value, Measurements, Recurring | Level 3 | Other contracts    
Assets:    
Assets, derivatives 8,000 5,000
Liabilities:    
Liabilities, derivatives 8,000 3,000
Fair Value, Measurements, Recurring | Level 3 | Japanese government bonds    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 3 | Japanese local government bonds    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury bonds and federal agency securities    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 3 | Other foreign government bonds    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 3 | Agency mortgage-backed securities    
Assets:    
Assets, trading securities [2] 0  
Available-for-sale securities 0  
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit and commercial paper    
Assets:    
Assets, trading securities [2] 0  
Fair Value, Measurements, Recurring | Level 3 | Corporate bonds and other    
Assets:    
Assets, trading securities [2],[4] 177,000 174,000
Fair Value, Measurements, Recurring | Level 3 | Equity securities    
Assets:    
Assets, trading securities [2] 21,000 21,000
Fair Value, Measurements, Recurring | Level 3 | Residential mortgage-backed securities    
Assets:    
Available-for-sale securities 12,000 6,000
Fair Value, Measurements, Recurring | Level 3 | Commercial mortgage-backed securities    
Assets:    
Available-for-sale securities 0 3,000
Fair Value, Measurements, Recurring | Level 3 | Japanese corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 228,000 186,000
Fair Value, Measurements, Recurring | Level 3 | Foreign corporate bonds and other debt securities    
Assets:    
Available-for-sale securities 0 13,000
Fair Value, Measurements, Recurring | Level 3 | Other Assets    
Assets:    
Other assets ¥ 40,000 ¥ 37,000 [5]
[1] In accordance with ASC 820, certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented for these classes of assets are intended to permit the reconciliation of the fair value hierarchy to the amounts presented in the statements of financial position. The amounts of unfunded commitments related to these investments at March 31, 2025 and 2026 were ¥58 billion and ¥190 billion, respectively.
[2] Trading securities include foreign currency denominated securities for which the MHFG Group elected the fair value option.
[3] Amounts represent items for which the Group elected the fair value option or for which it applied the practicability exception.
[4] The amount includes CLO and convertible bonds, which are classified in Level 3.
[5] The amount includes highly liquid government bonds classified as held for sale relating to a transferred business, which are classified in Level 1.