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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024 | Mar. 31, 2025 | Dec. 31, 2024
Derivative |  |  |  |  |  |
Credit Exposure on Derivatives | $ 0.0 |  | $ 0.0 |  |  | $ 0.5
Aggregate fair value of all derivative instruments with credit risk-related contingent features in a liability position | 259.6 |  | 259.6 |  |  | 255.7
Derivative, Notional Amount | 3,851.1 |  | 3,851.1 |  |  | 3,751.4
Approximate amount of net deferred gains on derivative instruments expected to be amortized during the next twelve months |  |  | 3.1 |  |  |
Component of Derivative Gain (Loss) Excluded from the Assessment of Hedge Effectiveness | 6.4 | $ 8.2 | 31.3 | $ 9.5 |  |
Discontinued Hedge Accounting Due to Instrument No Longer Qualifying as Fair Value Hedge | 0.0 | 0.0 | $ 0.0 | 0.0 |  |
Maximum Length of Time Hedged in Cash Flow Hedge |  |  | 28 years |  |  |
Net Gain (Loss) on Derivatives | (0.1) | 2.2 | $ 3.1 | 0.2 |  |
Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 3,532.3 |  | 3,532.3 |  |  | 3,445.5
Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 318.8 |  | 318.8 |  |  | 305.9
Net Gain (Loss) on Derivatives | (3.3) | 3.0 | 0.7 | (5.9) |  |
Cash Flow Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 2,763.6 |  | 2,763.6 |  |  | 2,709.1
Interest Rate Swaps | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 119.7 |  | 119.7 |  |  | 125.9
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 129.6 |  | 129.6 |  |  | 139.1
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument | Investment Income [Member] |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | (1.5) | (3.7) | (7.0) | (14.5) |  |
Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss) |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | (2.2) | 0.0 | (2.2) | 0.0 |  |
Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 768.7 |  | 768.7 |  |  | 736.4
Total Return Swap | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 150.0 |  | 150.0 |  |  | 128.9
Total Return Swap | Not Designated as Hedging Instrument | Investment Income [Member] |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | 0.0 | 0.0 | (0.9) | 0.0 |  |
Total Return Swap | Not Designated as Hedging Instrument | Net Investment Gain (Loss) |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | 0.0 | 0.0 | (2.8) | 0.0 |  |
Total Return Swap | Not Designated as Hedging Instrument | Other Expense |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | (6.8) | (5.4) | (13.8) | (14.1) |  |
Foreign Exchange Contracts | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | 0.0 | 2.2 | 5.9 | 0.2 |  |
Foreign Exchange Contracts | Cash Flow Hedging | Designated as Hedging Instrument | Investment Income [Member] |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | 0.9 | (0.9) | 1.3 | (0.5) |  |
Foreign Exchange Contracts | Cash Flow Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss) |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | (0.8) | (0.7) | (0.9) | (0.5) |  |
Foreign Exchange Contracts | Fair Value Hedging | Designated as Hedging Instrument | Investment Income [Member] |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | (3.7) | (4.2) | (1.4) | (11.5) |  |
Foreign Exchange Contracts | Fair Value Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss) |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | (2.3) | 25.2 | 69.8 | 5.6 |  |
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 0.0 |  | 0.0 |  |  | 0.0
Net Gain (Loss) on Derivatives | (3.3) | 0.8 | (2.4) | (6.1) |  |
Foreign Exchange Forward | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 49.1 |  | 49.1 |  |  | 51.1
Forward Benchmark Interest Rate Lock | Cash Flow Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 2,634.0 |  | 2,634.0 |  |  | 2,570.0
Forward Benchmark Interest Rate Lock | Cash Flow Hedging | Designated as Hedging Instrument | Investment Income [Member] |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | 0.4 | 0.3 | 1.2 | 0.7 |  |
Forward Benchmark Interest Rate Lock | Cash Flow Hedging | Designated as Hedging Instrument | Net Investment Gain (Loss) |  |  |  |  |  |
Derivative |  |  |  |  |  |
Net Gain (Loss) on Derivatives | 3.1 | $ 0.0 | 3.1 | $ 0.0 |  |
Receive Fixed/Pay Fixed | Interest Rate Swaps | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 119.7 |  | 119.7 |  |  | 125.9
Receive Fixed/Pay Fixed | Interest Rate Swaps | Cash Flow Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 129.6 |  | 129.6 |  |  | 139.1
Receive Fixed/Pay Fixed | Interest Rate Swaps | Fair Value Hedging | Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | 768.7 |  | 768.7 |  |  | 736.4
Credit Spreads and Interest Rate Volatility | Total Return Swap | Not Designated as Hedging Instrument |  |  |  |  |  |
Derivative |  |  |  |  |  |
Derivative, Notional Amount | $ 0.0 |  | $ 0.0 |  | $ 700.0 | $ 0.0

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- Definition The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480870/815-30-50-1 + Details
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- Definition Amount of increase (decrease) in the fair value of derivatives recognized in the income statement. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A Reference 2: http://www.xbrl.org/2003/role/exampleRef -Name Accounting Standards Codification -Section 55 -Paragraph 10 -SubTopic 10 -Topic 825 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482881/825-10-55-10 + Details
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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 185 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480401/815-10-55-185 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4H -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4H + Details
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