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Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Income Tax [Abstract] |  |  |
Income Tax Expense | $ 195.0 | $ 472.2 | $ 356.3
Total Income Tax Expense | 361.8 | 696.1 | 381.6
Other Comprehensive Income (Loss), Tax |  |  |
Income Tax Expense (Benefit), Change in Net Unrealized Gain on Securities | 200.4 | (226.6) | 300.6
Income Tax Expense (Benefit), Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy and Contract Benefits | (66.2) | 488.9 | (256.5)
Income Tax Benefit, Change in Net Gain (Loss) on Derivatives | (3.1) | (51.5) | (17.0)
Income Tax Expense (Benefit), Change in Foreign Currency Translation Adjustment | 0.1 | 1.1 | 0.9
Income Tax Expense (Benefit), Change in Unrecognized Pension and Postretirement Benefit Costs | $ 35.6 | $ 12.0 | $ (2.7)

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- Definition Amount of tax expense (benefit), after reclassification adjustments, of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of hedges. + References No definition available. + Details
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- Definition Amount, after adjustment, of tax expense (benefit) for unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and tax expense (benefit) for unrealized gain (loss) on investment in debt security measured at amortized cost (held-to-maturity) from transfer to available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 10A -Subparagraph (e) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 10A -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-10A Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482790/220-10-45-12 + Details
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