Fair value measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands |
Jun. 30, 2026 |
Dec. 31, 2025 |
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|---|---|---|---|---|
| Cash and due from banks | ||||
| Interest-bearing deposits at banks | $ 15,499,000 | $ 17,068,000 | ||
| Investment securities held to maturity | 38,374,000 | 36,649,000 | ||
| Net loans | 141,017,000 | 136,586,000 | ||
| Financial liabilities: | ||||
| Short-term borrowings | 64,000 | 49,000 | ||
| Loan facility carrying amount | [1] | 13,568,000 | 10,911,000 | |
| Time deposits above US insurance limit | 2,900,000 | 2,800,000 | ||
| Level 1 | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 1,746,000 | 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 | 193,000 | 113,000 | ||
| Interest-bearing deposits at banks | 15,499,000 | 17,068,000 | ||
| Investment securities held to maturity | 11,084,000 | 11,671,000 | ||
| Net loans | 4,316,000 | 7,427,000 | ||
| Financial liabilities: | ||||
| Time deposits | 14,946,000 | 13,208,000 | ||
| Short-term borrowings | 4,614,000 | 2,149,000 | ||
| Loan facility carrying amount | 13,785,000 | 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 | 39,000 | 44,000 | ||
| Net loans | 136,477,000 | 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 | 1,939,000 | 1,701,000 | ||
| Interest-bearing deposits at banks | 15,499,000 | 17,068,000 | ||
| Investment securities held to maturity | 11,908,000 | 12,430,000 | ||
| Net loans | 141,017,000 | 136,586,000 | ||
| Financial liabilities: | ||||
| Time deposits | 14,988,000 | 13,227,000 | ||
| Short-term borrowings | 4,614,000 | 2,149,000 | ||
| Loan facility carrying amount | 13,568,000 | 10,911,000 | ||
| Estimated Fair Value | ||||
| Cash and due from banks | ||||
| Cash and due from banks | 1,939,000 | 1,701,000 | ||
| Interest-bearing deposits at banks | 15,499,000 | 17,068,000 | ||
| Investment securities held to maturity | 11,123,000 | 11,715,000 | ||
| Net loans | 140,793,000 | 136,269,000 | ||
| Financial liabilities: | ||||
| Time deposits | 14,946,000 | 13,208,000 | ||
| Short-term borrowings | 4,614,000 | 2,149,000 | ||
| Loan facility carrying amount | $ 13,785,000 | $ 11,179,000 | ||
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