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Benefit Plans - Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 297,603 | $ 260,747 |
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 231,996 | 192,752 |
Significant Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 65,607 | 67,995 |
Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 183,509 | 157,050 |
Equity securities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 183,509 | 149,265 |
Equity securities | Significant Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 7,785 |
Equity securities | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Cash and equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 12,475 | 11,801 |
Cash and equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 12,475 | 11,801 |
Cash and equivalents | Significant Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Cash and equivalents | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Fixed income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 101,619 | 91,896 |
Fixed income | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 36,012 | 31,686 |
Fixed income | Significant Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 65,607 | 60,210 |
Fixed income | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 0 | 0 |
Pension Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | 278,124 | 243,521 | $ 127,610
Pension Plan | Debt securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 95,935 | $ 86,288 |
Actual plan asset allocations as a percent of total plan assets | 35.00% | 36.00% |
Pension Plan | Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 170,747 | $ 146,210 |
Actual plan asset allocations as a percent of total plan assets | 61.00% | 60.00% |
Pension Plan | Cash and equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 11,442 | $ 11,023 |
Actual plan asset allocations as a percent of total plan assets | 4.00% | 5.00% |
Other Postretirement Benefits Plan |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 19,479 | $ 17,226 | $ 5,849
Other Postretirement Benefits Plan | Debt securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 5,684 | $ 5,608 |
Actual plan asset allocations as a percent of total plan assets | 29.00% | 33.00% |
Other Postretirement Benefits Plan | Equity securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 12,762 | $ 10,840 |
Actual plan asset allocations as a percent of total plan assets | 66.00% | 63.00% |
Other Postretirement Benefits Plan | Cash and equivalents |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Fair value of plan assets | $ 1,033 | $ 778 |
Actual plan asset allocations as a percent of total plan assets | 5.00% | 5.00% |

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