| **Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) - USD ($) $ in Thousands** | 3 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------|----------------|
||
| **Accumulated Other Comprehensive Income (Loss)**                                                                            |           |   |
| Beginning balance                                                                                                            | $ 1,408,260    | $ 1,303,463 |
| Other comprehensive income before reclassifications                                                                          | 20,681         | (17,054) |
| Amounts reclassified from accumulated other comprehensive income                                                             | (854)          | (1,137) |
| Total other comprehensive income (loss), net of tax                                                                          | 19,827         | (18,191) |
| Reclassification adjustment under ASU 2018-02                                                                                |           | 0 |
| Ending balance                                                                                                               | 1,455,848      | 1,313,073 |
| Total                                                                                                                        |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                                            |           |   |
| Beginning balance                                                                                                            | (21,422)       | (2,908) |
| Total other comprehensive income (loss), net of tax                                                                          | 19,827         | (18,191) |
| Reclassification adjustment under ASU 2018-02                                                                                | (626)          | (626) |
| Ending balance                                                                                                               | (1,595)        | (21,725) |
| Unrealized Gains (Losses) on Securities Available for Sale                                                                   |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                                            |           |   |
| Beginning balance                                                                                                            | (6,343)        | 8,970 |
| Other comprehensive income before reclassifications                                                                          | 20,990         | (17,698) |
| Amounts reclassified from accumulated other comprehensive income                                                             | (901)          | (1,271) |
| Total other comprehensive income (loss), net of tax                                                                          | 20,089         | (18,969) |
| Reclassification adjustment under ASU 2018-02                                                                                | 1,932          |   |
| Ending balance                                                                                                               | 13,746         | (8,067) |
| Unrealized Gains (Losses) on Cash Flow Hedges                                                                                |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                                            |           |   |
| Beginning balance                                                                                                            | (559)          | (1,125) |
| Other comprehensive income before reclassifications                                                                          | (309)          | 644 |
| Amounts reclassified from accumulated other comprehensive income                                                             | 47             | 134 |
| Total other comprehensive income (loss), net of tax                                                                          | (262)          | 778 |
| Reclassification adjustment under ASU 2018-02                                                                                | (242)          |   |
| Ending balance                                                                                                               | (821)          | (589) |
| Unrealized Gains (Losses) on Defined Benefit Plans                                                                           |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                                            |           |   |
| Beginning balance                                                                                                            | (14,520)       | (10,753) |
| Other comprehensive income before reclassifications                                                                          | 0              | 0 |
| Amounts reclassified from accumulated other comprehensive income                                                             | 0              | 0 |
| Total other comprehensive income (loss), net of tax                                                                          | 0              | 0 |
| Reclassification adjustment under ASU 2018-02                                                                                | (2,316)        |   |
| Ending balance                                                                                                               | $ (14,520)     | $ (13,069) |

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|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect
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| - Definition
Amount after tax, before reclassification adjustments of other comprehensive income (loss).
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| - Definition
Amount after tax and reclassification adjustments of other comprehensive income (loss).
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| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss).
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| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
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