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Pension Plans and Postretirement Benefits - Summary of Total Investments Measured at Fair Value (Detail) - USD ($) $ in Thousands | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
--- | --- | --- | ---
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | $ 926,395 | $ 921,307 |
Unclassified | 554,655 | 537,569 |
U.S. Government Securities |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 288,495 | 305,995 |
Unclassified | 0 | 0 |
Commingled and Mutual Funds |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 231,801 | 230,674 |
Unclassified | 185,297 | 188,494 |
Real Estate |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 123,250 | 132,867 |
Unclassified | 123,250 | 132,867 |
Diversified Credit |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 113,271 | 108,363 |
Unclassified | 113,271 | 108,363 |
Fixed Income |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 108,646 | 95,967 |
Unclassified | 71,905 | 60,404 |
Partnerships with Security Holdings |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 19,752 | 25,259 |
Unclassified | 19,752 | 25,259 |
Cash, Cash Equivalents and Accrued Items (1) |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 41,180 | 22,182 |
Unclassified | 41,180 | 22,182 |
Level 1 |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 371,740 | 383,738 |
Level 1 | U.S. Government Securities |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 288,495 | 305,995 |
Level 1 | Commingled and Mutual Funds |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 46,504 | 42,180 |
Level 1 | Real Estate |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 1 | Diversified Credit |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 1 | Fixed Income |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 36,741 | 35,563 |
Level 1 | Partnerships with Security Holdings |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 1 | Cash, Cash Equivalents and Accrued Items (1) |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | U.S. Government Securities |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | Commingled and Mutual Funds |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | Real Estate |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | Diversified Credit |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | Fixed Income |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | Partnerships with Security Holdings |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 2 | Cash, Cash Equivalents and Accrued Items (1) |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 | $ 46,817
Level 3 | U.S. Government Securities |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 | Commingled and Mutual Funds |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 | Real Estate |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 | Diversified Credit |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 | Fixed Income |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 | Partnerships with Security Holdings |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | 0 | 0 |
Level 3 | Cash, Cash Equivalents and Accrued Items (1) |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |
Total Assets | $ 0 | $ 0 |

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