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Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details) - Retirement Income Plan [Member] - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | $ 432,716 | $ 385,087 | $ 331,680
Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 435,761 | 386,920 |
Total Equities [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 277,667 | 229,635 |
Return Seeking Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 278,169 | 230,489 |
Liability Hedging Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 152,246 | 145,392 |
Cash And Short Term Investments [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 5,346 | 11,039 |
Alternative Investments [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 502 | 854 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 433,186 | 383,851 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Total Equities [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 277,667 | 229,635 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Return Seeking Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 277,667 | 229,635 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Liability Hedging Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 152,246 | 145,392 |
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Cash And Short Term Investments [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 3,273 | 8,824 |
Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 2,073 | 2,215 |
Significant Other Observable Inputs (Level 2) [Member] | Total Equities [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Other Observable Inputs (Level 2) [Member] | Return Seeking Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Other Observable Inputs (Level 2) [Member] | Liability Hedging Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Other Observable Inputs (Level 2) [Member] | Cash And Short Term Investments [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 2,073 | 2,215 |
Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Total Equities [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Return Seeking Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Liability Hedging Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Significant Unobservable Inputs (Level 3) [Member] | Cash And Short Term Investments [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Global Equity [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 142,320 | 113,212 |
Global Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 142,320 | 113,212 |
Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Diversified Credit [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 73,762 | 59,009 |
Diversified Credit [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 73,762 | 59,009 |
Diversified Credit [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Diversified Credit [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Real Assets Equity [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 61,585 | 57,414 |
Real Assets Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 61,585 | 57,414 |
Real Assets Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Real Assets Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Real Assets [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 73 | 228 |
Real Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Real Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Real Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Private Equity Funds [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 400 | 583 |
Private Equity Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Private Equity Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Private Equity Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Private Credit Funds [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 29 | 43 |
Private Credit Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Private Credit Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Private Credit Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Alternative Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Alternative Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Alternative Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets | [1] | 0 | 0 |
Fixed Income Securities [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 99,490 | 114,395 |
Fixed Income Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 99,490 | 114,395 |
Fixed Income Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Fixed Income Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
US Treasury Securities [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 52,756 | 30,997 |
US Treasury Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 52,756 | 30,997 |
US Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
US Treasury Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Short-term Investments [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 3,273 | 8,824 |
Short-term Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 3,273 | 8,824 |
Short-term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Short-term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Deposit Administration Contracts [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 2,073 | 2,215 |
Deposit Administration Contracts [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 0 | 0 |
Deposit Administration Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | 2,073 | 2,215 |
Deposit Administration Contracts [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Assets |  | $ 0 | $ 0 |
[1] | In accordance with ASU 2015-07, certain investments that are measured at fair value using the net asset value per share (or its practical expedient) have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total Pension Plan invested assets.

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