| Segment Reporting, Reconciliation of Asset by Segment to Consolidated [Table Text Block] |
| | | Trust | | | Bank | | | Consolidated | |
| (In Thousands of Dollars) | | Segment | | | Segment | | | Segment totals | |
| March 31, 2026 | | | | | | | | | | | | |
| Total assets for reportable segments | | $ | 17,678 | | | $ | 7,215,696 | | | $ | 7,233,374 | |
| Eliminations and other | | | | | | | | | | | (57,898 | ) |
| Total consolidated assets | | | | | | | | | | $ | 7,175,476 | |
| | | Trust | | | Bank | | | Consolidated | |
| (In Thousands of Dollars) | | Segment | | | Segment | | | Segment totals | |
| December 31, 2025 | | | | | | | | | | | | |
| Total assets for reportable segments | | $ | 17,000 | | | $ | 5,230,175 | | | $ | 5,247,175 | |
| Eliminations and other | | | | | | | | | | | (1,305 | ) |
| Total consolidated assets | | | | | | | | | | $ | 5,245,870 | |
|
| Segment Reporting [Table Text Block] |
| | | Trust | | | Bank | | | Consolidated | |
| (In Thousands of Dollars) | | Segment | | | Segment | | | Segment totals | |
| For Three Months Ended March 31, 2026 | | | | | | | | | | | | |
| Interest income - loans including fees | | $ | 0 | | | $ | 55,109 | | | $ | 55,109 | |
| Interest income - investments | | | 0 | | | | 10,544 | | | | 10,544 | |
| Trust fees | | | 3,030 | | | | 0 | | | | 3,030 | |
| Retirement plan consulting fees | | | 886 | | | | 0 | | | | 886 | |
| Total consolidated segment revenues | | | 3,916 | | | | 65,653 | | | | 69,569 | |
| Reconciliation of revenue | | | | | | | | | | | | |
| Other revenues | | | | | | | | | | | 11,236 | |
| Total consolidated revenues | | | | | | | | | | | 80,805 | |
| | | | | | | | | | | | | |
| Interest expense - deposits | | | 0 | | | | 20,440 | | | | 20,440 | |
| Interest expense - borrowings | | | 0 | | | | 4,109 | | | | 4,109 | |
| Credit for credit losses and unfunded loans | | | 0 | | | | (1,034 | ) | | | (1,034 | ) |
| Payroll expenses | | | 1,679 | | | | 16,810 | | | | 18,489 | |
| Total consolidated segment expenses | | | 1,679 | | | | 40,325 | | | | 42,004 | |
| | | | | | | | | | | | | |
| Segment profit | | | 2,237 | | | | 25,328 | | | | 27,565 | |
| Reconciliation of expenses | | | | | | | | | | | | |
| Other expenses * | | | | | | | | | | | 18,829 | |
| Total consolidated expenses | | | | | | | | | | | 60,833 | |
| | | | | | | | | | | | | |
| Total consolidated income before taxes | | | | | | | | | | $ | 19,972 | |
| Other segment disclosures | | | | | | | | | | | | |
| Occupancy and equipment | | | 134 | | | | 4,961 | | | | 5,095 | |
| Intangible amortization | | | 14 | | | | 851 | | | | 865 | |
| | | Trust | | | Bank | | | Consolidated | |
| (In Thousands of Dollars) | | Segment | | | Segment | | | Segment totals | |
| For Three Months Ended March 31, 2025 | | | | | | | | | | | | |
| Interest income - loans including fees | | $ | 0 | | | $ | 46,707 | | | $ | 46,707 | |
| Interest income - investments | | | 0 | | | | 9,514 | | | | 9,514 | |
| Trust fees | | | 2,641 | | | | 0 | | | | 2,641 | |
| Retirement plan consulting fees | | | 798 | | | | 0 | | | | 798 | |
| Total consolidated segment revenues | | | 3,439 | | | | 56,221 | | | | 59,660 | |
| Reconciliation of revenue | | | | | | | | | | | | |
| Other revenues | | | | | | | | | | | 8,126 | |
| Total consolidated revenues | | | | | | | | | | | 67,786 | |
| | | | | | | | | | | | | |
| Interest expense - deposits | | | 0 | | | | 19,717 | | | | 19,717 | |
| Interest expense - borrowings | | | 0 | | | | 3,393 | | | | 3,393 | |
| Provision for credit losses and unfunded loans | | | 0 | | | | (204 | ) | | | (204 | ) |
| Payroll expenses | | | 1,474 | | | | 14,681 | | | | 16,155 | |
| Total consolidated segment expenses | | | 1,474 | | | | 37,587 | | | | 39,061 | |
| | | | | | | | | | | | | |
| Segment profit | | | 1,965 | | | | 18,634 | | | | 20,599 | |
| Reconciliation of expenses | | | | | | | | | | | | |
| Other expenses * | | | | | | | | | | | 12,371 | |
| Total consolidated expenses | | | | | | | | | | | 51,432 | |
| | | | | | | | | | | | | |
| Total consolidated income before taxes | | | | | | | | | | $ | 16,354 | |
| Other segment disclosures | | | | | | | | | | | | |
| Occupancy and equipment | | | 143 | | | | 3,983 | | | | 4,126 | |
| Intangible amortization | | | 23 | | | | 712 | | | | 735 | |
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