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Benefit Plans (Fair values of our pension plan assets by asset category) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | $ 8,263 | $ 8,352 | $ 7,765
Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | 8,263 | 8,352 |
Short-term investments [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | 69 | 218 |
Short-term investments [Member] | Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | 69 | 218 |
Fixed income [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | 3,851 | 3,823 |
Fixed income [Member] | Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | 3,851 | 3,823 |
Equity investments [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | 4,343 | 4,311 |
Equity investments [Member] | Level 2 [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair values of the Corporation's pension plan assets | $ 4,343 | $ 4,311 |

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