| **Accumulated Other Comprehensive Income (Loss) - Summary of Changes (Details) - USD ($) $ in Thousands** | 3 Months Ended |
|-----------------------------------------------------------------------------------------------------------|----------------|
||
| **Accumulated Other Comprehensive Income (Loss)**                                                         |           |   |
| Beginning balance                                                                                         | $ 1,912,571    | $ 1,875,645 |
| Other comprehensive income (loss) before reclassifications                                                | (139,248)      | (37,804) |
| Amounts reclassified from accumulated other comprehensive income                                          | (257)          | (1,222) |
| Total other comprehensive income (loss), net of tax                                                       | (139,505)      | (39,026) |
| Ending balance                                                                                            | 1,807,633      | 1,805,856 |
| Total                                                                                                     |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                         |           |   |
| Beginning balance                                                                                         | 55,113         | 74,836 |
| Total other comprehensive income (loss), net of tax                                                       | (139,505)      | (39,026) |
| Ending balance                                                                                            | (84,392)       | 35,810 |
| Unrealized Gains (Losses) on Securities Available for Sale                                                |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                         |           |   |
| Beginning balance                                                                                         | 59,774         | 87,988 |
| Other comprehensive income (loss) before reclassifications                                                | (139,487)      | (37,850) |
| Amounts reclassified from accumulated other comprehensive income                                          | (447)          | (1,421) |
| Total other comprehensive income (loss), net of tax                                                       | (139,934)      | (39,271) |
| Ending balance                                                                                            | (80,160)       | 48,717 |
| Unrealized Gains (Losses) on Cash Flow Hedges                                                             |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                         |           |   |
| Beginning balance                                                                                         | (660)          | (1,594) |
| Other comprehensive income (loss) before reclassifications                                                | 239            | 46 |
| Amounts reclassified from accumulated other comprehensive income                                          | 190            | 199 |
| Total other comprehensive income (loss), net of tax                                                       | 429            | 245 |
| Ending balance                                                                                            | (231)          | (1,349) |
| Unrealized Gains (Losses) on Defined Benefit Plans                                                        |           |   |
| **Accumulated Other Comprehensive Income (Loss)**                                                         |           |   |
| Beginning balance                                                                                         | (4,001)        | (11,558) |
| Other comprehensive income (loss) before reclassifications                                                | 0              | 0 |
| Amounts reclassified from accumulated other comprehensive income                                          | 0              | 0 |
| Total other comprehensive income (loss), net of tax                                                       | 0              | 0 |
| Ending balance                                                                                            | $ (4,001)      | $ (11,558) |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.
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| - Definition
Amount after tax of other comprehensive income (loss) attributable to parent entity.
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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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