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Pension and Other Postretirement Benefit Plans - Assets that are Carried at Fair Value, and the Method Used to Determine Fair Value (Details) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Long Corporate Bond Fund [Member] |  |  |
Pooled Separate Accounts |  |  |
Average duration of bonds | 3 months |  |
Retirement Plan [Member] |  |  |
Pooled Separate Accounts |  |  |
Plan Investments | $ 27,720 | $ 25,505 | $ 22,419
Retirement Plan [Member] | Long Duration Bond Fund [Member] |  |  |
Pooled Separate Accounts |  |  |
Plan Investments | 12,229 | 11,242 |
Retirement Plan [Member] | Long Corporate Bond Fund [Member] |  |  |
Pooled Separate Accounts |  |  |
Plan Investments | 5,587 | 5,069 |
Retirement Plan [Member] | Prudential Short-term [Member] |  |  |
Pooled Separate Accounts |  |  |
Plan Investments | 286 | 403 |
Retirement Plan [Member] | Investment Grade Bond Fund [Member] |  |  |
Pooled Separate Accounts |  |  |
Plan Investments | $ 9,618 | $ 8,791 |

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