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Benefit Plans - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | $ 402,691 | $ 359,719 |
Pension Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 39,491 | 38,889 |
Pension Plan | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 363,200 | 320,830 |
Pension Plan | Domestic Mutual Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 7,325 | 13,744 |
Pension Plan | Domestic Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 7,325 | 13,744 |
Pension Plan | Domestic Mutual Funds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Corporate Bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 203,684 | 181,363 |
Pension Plan | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Corporate Bonds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 203,684 | 181,363 |
Pension Plan | U.S. Government Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 34,677 | 21,228 |
Pension Plan | U.S. Government Securities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 32,166 | 19,904 |
Pension Plan | U.S. Government Securities | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 2,511 | 1,324 |
Pension Plan | Municipal Bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 38,998 | 43,424 |
Pension Plan | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Municipal Bonds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 38,998 | 43,424 |
Pension Plan | Foreign Bonds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 34,024 | 30,113 |
Pension Plan | Foreign Bonds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Foreign Bonds | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 34,024 | 30,113 |
Pension Plan | Exchange traded fund |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 5,241 |
Pension Plan | Exchange traded fund | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 5,241 |
Pension Plan | Exchange traded fund | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Pooled Equity Accounts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 83,983 | 64,606 |
Pension Plan | Pooled Equity Accounts | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 0 | 0 |
Pension Plan | Pooled Equity Accounts | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 83,983 | 64,606 |
Other Postretirement Benefit Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 99,590 | 77,762 | $ 85,303
Other Postretirement Benefit Plan | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 99,590 | 77,762 |
Other Postretirement Benefit Plan | Domestic Mutual Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 77,640 | 60,405 |
Other Postretirement Benefit Plan | Domestic Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 77,640 | 60,405 |
Other Postretirement Benefit Plan | International Mutual Funds |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | 21,950 | 17,357 |
Other Postretirement Benefit Plan | International Mutual Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Plan Assets at Fair Value | $ 21,950 | $ 17,357 |

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