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Investments - Summary of Fair Value and Amortized Costs (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | $ 5,652.9 | $ 5,756.9
Fixed maturity securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 5,652.9 | 5,756.9
Gross Unrealized Gains | 62.9 | 35.0
Gross Unrealized Losses | 480.5 | 606.9
Fair Value | 5,235.3 | 5,185.0
Mortgage-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 713.4 | 638.2
Gross Unrealized Gains | 4.4 | 1.3
Gross Unrealized Losses | 64.6 | 69.1
Fair Value | 653.2 | 570.4
Other, including U.S. Treasury securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 450.8 | 410.0
Gross Unrealized Gains | 0.8 | 0.5
Gross Unrealized Losses | 62.8 | 67.8
Fair Value | 388.8 | 342.7
Municipal bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 1,333.4 | 1,380.9
Gross Unrealized Gains | 28.6 | 16.9
Gross Unrealized Losses | 91.9 | 128.1
Fair Value | 1,270.1 | 1,269.7
Foreign government bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 23.1 | 35.1
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 1.0 | 1.6
Fair Value | 22.1 | 33.5
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 1,969.9 | 2,161.2
Gross Unrealized Gains | 23.1 | 12.7
Gross Unrealized Losses | 220.3 | 272.2
Fair Value | 1,772.7 | 1,901.7
Other asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost, net | 1,162.3 | 1,131.5
Gross Unrealized Gains | 6.0 | 3.6
Gross Unrealized Losses | 39.9 | 68.1
Fair Value | $ 1,128.4 | $ 1,067.0

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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