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Pension Plans and Postretirement Benefits - Summary of Total Investments Measured at Fair Value (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | $ 1,333,309 | $ 1,198,816
Unclassified | 715,313 | 541,162
Fixed Income |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 391,418 | 453,390
Unclassified | 162,561 | 98,326
Commingled and Mutual Funds |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 338,473 | 334,786
Unclassified | 265,709 | 276,625
U.S. Government Securities |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 316,375 | 220,606
Unclassified | 0 | 0
Other |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 126,823 | 47,550
Unclassified | 126,823 | 47,550
Real Estate |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 77,437 | 65,821
Unclassified | 77,437 | 65,821
Partnerships with Security Holdings |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 49,007 | 52,840
Unclassified | 49,007 | 52,840
Cash and Accrued Items (1) |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 33,776 | 23,823
Unclassified | 33,776 | 0
Level 1 |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 383,739 | 291,641
Level 1 | Fixed Income |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 1 | Commingled and Mutual Funds |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 72,764 | 58,161
Level 1 | U.S. Government Securities |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 310,975 | 216,300
Level 1 | Other |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 1 | Real Estate |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 1 | Partnerships with Security Holdings |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 1 | Cash and Accrued Items (1) |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 17,180
Level 2 |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 234,257 | 366,013
Level 2 | Fixed Income |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 228,857 | 355,064
Level 2 | Commingled and Mutual Funds |  |
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 | 5,400 | 4,306
Level 2 | Other |  |
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 | Partnerships with Security Holdings |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 2 | Cash and Accrued Items (1) |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 6,643
Level 3 |  |
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 | Commingled and Mutual Funds |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 3 | U.S. Government Securities |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 3 | Other |  |
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 | Partnerships with Security Holdings |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | 0 | 0
Level 3 | Cash and Accrued Items (1) |  |
Fair Value, Separate Account Investment [Line Items] |  |
Total Assets | $ 0 | $ 0

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