| **CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands**                                                                                               | 12 Months Ended |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
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| **Statement of Comprehensive Income [Abstract]**                                                                                                                           |            |   |   |
| Net income                                                                                                                                                                 | $ 65,384        | $ 60,162 | $ 44,530 |
| **Other comprehensive income (loss) net of tax:**                                                                                                                          |            |   |   |
| Unrealized holding gain (loss) on securities available for sale arising during the period, net of tax of $20, $8,001, and $6,841                                           | $ (37)          | $ 14,860 | (12,705) |
| Unrealized loss on securities transferred to held-to-maturity, net of tax of $0, $0, and $1,786                                                                            |                 |  | (3,316) |
| Unrealized gain on securities available for sale for which a portion of an other than temporary impairment has been recognized in income, net of tax of $0, $995, and $767 |                 | $ 1,847 | 1,425 |
| Unrealized gain (loss) on cash flow hedges arising during the period, net of tax of $588, $1,397, and $831                                                                 | $ (1,093)       | (2,599) | 1,543 |
| Reclassification adjustment for net losses (gains) included in net income net of tax of $435, $760, and $157                                                               | (808)           | (1,410) | 291 |
| **Defined benefit pension plans, net of tax of $1,187, $4,264, and $6,382**                                                                                                |            |   |   |
| Net gain (loss) arising during period                                                                                                                                      | 2,404           | (7,580) | 10,704 |
| Amortization of prior service cost                                                                                                                                         | (198)           | (338) | 1,147 |
| Period change                                                                                                                                                              | 268             | 4,780 | (911) |
| Comprehensive income                                                                                                                                                       | $ 65,652        | $ 64,942 | $ 43,619 |

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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Other Comprehensive Income Unrealized Gain (Loss) On Securities Transferred To Held To Maturity Arising During Period Net Of Tax
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| - Definition
Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.
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| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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| - Definition
Amount after tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.
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| - Definition
Amount of tax (expense) benefit of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans.
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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 adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.
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| - Definition
Amount after tax of other than temporary impairment (OTTI) on investment in available-for-sale debt security, recognized in other comprehensive loss, attributable to parent entity.
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