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Retirement Plans - Reconciliation of Beginning and Ending Balances for Level 3 Assets of Plan Measured at Fair Value (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets at beginning of year | $ 212.5 | $ 282.3
Fair value of plan assets at end of year | 228.9 | 212.5
Level 3 [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Fair value of plan assets at beginning of year | 99.6 | 124.2
Settlements | (1.4) | (1.5)
Unrealized gain | 10.4 | (23.1)
Fair value of plan assets at end of year | $ 108.6 | $ 99.6

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- Definition Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(8) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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