Fair value measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands |
Mar. 31, 2026 |
Dec. 31, 2025 |
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|---|---|---|---|---|
| Cash and due from banks | ||||
| Interest-bearing deposits at banks | $ 14,445,000 | $ 17,068,000 | ||
| Investment securities held to maturity | 38,621,000 | 36,649,000 | ||
| Net loans | 137,778,000 | 136,586,000 | ||
| Financial liabilities: | ||||
| Short-term borrowings | 51,000 | 49,000 | ||
| Loan facility carrying amount | [1] | 11,175,000 | 10,911,000 | |
| Time deposits above US insurance limit | 2,700,000 | 2,800,000 | ||
| Level 1 | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 1,732,000 | 1,588,000 | ||
| Interest-bearing deposits at banks | 0 | 0 | ||
| Investment securities held to maturity | 0 | 0 | ||
| Net loans | 0 | 0 | ||
| Financial liabilities: | ||||
| Time deposits | 0 | 0 | ||
| Short-term borrowings | 0 | 0 | ||
| Loan facility carrying amount | 0 | 0 | ||
| Level 2 | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 171,000 | 113,000 | ||
| Interest-bearing deposits at banks | 14,445,000 | 17,068,000 | ||
| Investment securities held to maturity | 11,314,000 | 11,671,000 | ||
| Net loans | 4,786,000 | 7,427,000 | ||
| Financial liabilities: | ||||
| Time deposits | 13,051,000 | 13,208,000 | ||
| Short-term borrowings | 7,851,000 | 2,149,000 | ||
| Loan facility carrying amount | 11,327,000 | 11,179,000 | ||
| Level 3 | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 0 | 0 | ||
| Interest-bearing deposits at banks | 0 | 0 | ||
| Investment securities held to maturity | 41,000 | 44,000 | ||
| Net loans | 132,443,000 | 128,842,000 | ||
| Financial liabilities: | ||||
| Time deposits | 0 | 0 | ||
| Short-term borrowings | 0 | 0 | ||
| Loan facility carrying amount | 0 | 0 | ||
| Carrying Amount | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 1,903,000 | 1,701,000 | ||
| Interest-bearing deposits at banks | 14,445,000 | 17,068,000 | ||
| Investment securities held to maturity | 12,119,000 | 12,430,000 | ||
| Net loans | 137,778,000 | 136,586,000 | ||
| Financial liabilities: | ||||
| Time deposits | 13,082,000 | 13,227,000 | ||
| Short-term borrowings | 7,851,000 | 2,149,000 | ||
| Loan facility carrying amount | 11,175,000 | 10,911,000 | ||
| Estimated Fair Value | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 1,903,000 | 1,701,000 | ||
| Interest-bearing deposits at banks | 14,445,000 | 17,068,000 | ||
| Investment securities held to maturity | 11,355,000 | 11,715,000 | ||
| Net loans | 137,229,000 | 136,269,000 | ||
| Financial liabilities: | ||||
| Time deposits | 13,051,000 | 13,208,000 | ||
| Short-term borrowings | 7,851,000 | 2,149,000 | ||
| Loan facility carrying amount | $ 11,327,000 | $ 11,179,000 | ||
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