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Investments - Net Unrealized Investment Gains and Losses on Fixed Maturities and Equity Securities (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance | [1] | $ 1,098.3 | $ 1,499.0 |  |  |
Ending balance |  | 1,175.3 | 1,098.3 | [1] | $ 1,499.0 | [1]
Net unrealized investment gains (losses) on securities |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  | (449.6) | 347.1 |  | 366.3 |
Ending balance |  | (328.3) | (449.6) |  | 347.1 |
Net unrealized investment gains (losses) on securities | Effect of adopting ASU 2018-12 |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  |  |  |  | 71.1 |
Net unrealized investment gains (losses) on securities | Fixed maturity securities |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  | (449.6) | 347.1 |  | 366.3 |
Change in net unrealized investment gains (losses) on fixed maturity securities |  | 105.3 | (819.7) |  | (96.4) |
Reclassification of net investment (gains) losses on fixed maturity securities to net income |  | 16.0 | 23.0 |  | 6.1 |
Ending balance |  | $ (328.3) | $ (449.6) |  | 347.1 |
Net unrealized investment gains (losses) on securities | Fixed maturity securities | Effect of adopting ASU 2018-12 |  |  |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |  |  |
Beginning balance |  |  |  |  | $ 71.1 |
[1] | Recast for the adoption of ASU 2018-12. See Note 1 of the Consolidated Financial Statements

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- Definition Amount, after tax and adjustment, of 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 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 11 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482790/220-10-45-11 + Details
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