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Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Pretax total |  | $ (337.4) | $ (82.9) | $ (142.5)
Unrealized Gains (Losses) on Debt Securities |  |  |  |
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Net realized losses on sales of debt securities |  | (335.1) | (80.9) | (136.5)
Tax effect |  | 89.5 | 19.8 | 36.2
Pension Benefit Adjustment |  |  |  |
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] |  |  |  |
Amortization of net actuarial loss | [1] | (2.3) | (2.0) | (5.8)
Amortization of prior service cost | [1] | 0.0 | 0.0 | (0.2)
Pretax total | [1] | (2.3) | (2.0) | (6.0)
Tax effect | [1] | $ 0.6 | $ 0.5 | $ 1.6
[1] | Amounts are components of net periodic cost. See Note 16 Employee Benefit Plans for additional details.

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- Definition Amount before tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 45 -Paragraph 15 -SubTopic 10 -Topic 220 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-15 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-11 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-9 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 17A -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-17A + Details
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