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Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Investment securities |  |
Common equity securities, at fair value | $ 538.4 | $ 668.4
Other long-term investments | 1,998.2 | 1,488.0
U.S. Government and agency obligations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 204.9 | 206.4
Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 1,045.0 | 1,018.8
Municipal obligations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 260.9 | 258.6
Mortgage and asset-backed securities |  |
Investment securities |  |
Fixed maturity investments, at fair value | 389.4 | 254.2
Mortgage and asset-backed securities | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 389.4 | 254.2
Mortgage and asset-backed securities | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Collateralized loan obligations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 209.1 | 182.9
Collateralized loan obligations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 209.1 | 182.9
Collateralized loan obligations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fixed maturity investments |  |
Investment securities |  |
Fixed maturity investments, at fair value | 2,109.3 | 1,920.9
Fair value measured on a recurring basis |  |
Investment securities |  |
Fair value investments | 6,388.7 | 5,170.0
Fair value measured on a recurring basis | MediaAlpha |  |
Investment securities |  |
Other long-term investments | 254.9 | 168.6
Fair value measured on a recurring basis | Level 1 |  |
Investment securities |  |
Fair value investments | 2,055.2 | 1,632.9
Fair value measured on a recurring basis | Level 1 | MediaAlpha |  |
Investment securities |  |
Other long-term investments | 254.9 | 168.6
Fair value measured on a recurring basis | Level 2 |  |
Investment securities |  |
Fair value investments | 2,361.5 | 2,063.9
Fair value measured on a recurring basis | Level 2 | MediaAlpha |  |
Investment securities |  |
Other long-term investments | 0.0 | 0.0
Fair value measured on a recurring basis | Level 3 |  |
Investment securities |  |
Fair value investments | 1,138.2 | 911.6
Fair value measured on a recurring basis | Level 3 | MediaAlpha |  |
Investment securities |  |
Other long-term investments | 0.0 | 0.0
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 204.9 | 206.4
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 204.9 | 206.4
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 1,045.0 | 1,018.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 1,045.0 | 1,018.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Municipal obligations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 260.9 | 258.6
Fair value measured on a recurring basis | Municipal obligations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Municipal obligations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 260.9 | 258.6
Fair value measured on a recurring basis | Municipal obligations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Mortgage and asset-backed securities |  |
Investment securities |  |
Fixed maturity investments, at fair value | 389.4 | 254.2
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 389.4 | 254.2
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Collateralized loan obligations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 209.1 | 182.9
Fair value measured on a recurring basis | Collateralized loan obligations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Collateralized loan obligations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 209.1 | 182.9
Fair value measured on a recurring basis | Collateralized loan obligations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Fixed maturity investments |  |
Investment securities |  |
Fixed maturity investments, at fair value | 2,109.3 | 1,920.9
Fair value measured on a recurring basis | Fixed maturity investments | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 204.9 | 206.4
Fair value measured on a recurring basis | Fixed maturity investments | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 1,904.4 | 1,714.5
Fair value measured on a recurring basis | Fixed maturity investments | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Short-term investments |  |
Investment securities |  |
Short-term investments, at fair value | 1,487.9 | 924.1
Fair value measured on a recurring basis | Short-term investments | Level 1 |  |
Investment securities |  |
Short-term investments, at fair value | 1,457.6 | 924.1
Fair value measured on a recurring basis | Short-term investments | Level 2 |  |
Investment securities |  |
Short-term investments, at fair value | 30.3 | 0.0
Fair value measured on a recurring basis | Short-term investments | Level 3 |  |
Investment securities |  |
Short-term investments, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Exchange-traded funds |  |
Investment securities |  |
Common equity securities, at fair value | 137.8 | 333.8
Fair value measured on a recurring basis | Exchange-traded funds | Level 1 |  |
Investment securities |  |
Common equity securities, at fair value | 137.8 | 333.8
Fair value measured on a recurring basis | Exchange-traded funds | Level 2 |  |
Investment securities |  |
Common equity securities, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Exchange-traded funds | Level 3 |  |
Investment securities |  |
Common equity securities, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Common stock, other |  |
Investment securities |  |
Common equity securities, at fair value | 400.6 | 334.6
Fair value measured on a recurring basis | Common stock, other | Level 1 |  |
Investment securities |  |
Common equity securities, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Common stock, other | Level 2 |  |
Investment securities |  |
Common equity securities, at fair value | 400.6 | 334.6
Fair value measured on a recurring basis | Common stock, other | Level 3 |  |
Investment securities |  |
Common equity securities, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Common equity securities |  |
Investment securities |  |
Common equity securities, at fair value | 538.4 | 668.4
Fair value measured on a recurring basis | Common equity securities | Level 1 |  |
Investment securities |  |
Common equity securities, at fair value | 137.8 | 333.8
Fair value measured on a recurring basis | Common equity securities | Level 2 |  |
Investment securities |  |
Common equity securities, at fair value | 400.6 | 334.6
Fair value measured on a recurring basis | Common equity securities | Level 3 |  |
Investment securities |  |
Common equity securities, at fair value | 0.0 | 0.0
Fair value measured on a recurring basis | Other long-term investments | Fair Value, Inputs, Level 1, Level 2, and Level 3 |  |
Investment securities |  |
Other long-term investments | 1,164.4 | 926.4
Fair value measured on a recurring basis | Other long-term investments | Level 1 |  |
Investment securities |  |
Other long-term investments | 0.0 | 0.0
Fair value measured on a recurring basis | Other long-term investments | Level 2 |  |
Investment securities |  |
Other long-term investments | 26.2 | 14.8
Fair value measured on a recurring basis | Other long-term investments | Level 3 |  |
Investment securities |  |
Other long-term investments | 1,138.2 | 911.6
Fair value measured on a recurring basis | Other long-term investments | NAV |  |
Investment securities |  |
Other long-term investments | 833.8 | 561.6
Fair value measured on a recurring basis | Other Long-Term Investments |  |
Investment securities |  |
Other long-term investments | 1,998.2 | 1,488.0
Fair value measured on a recurring basis | Other Long-Term Investments | Level 1 |  |
Investment securities |  |
Other long-term investments | 0.0 | 0.0
Fair value measured on a recurring basis | Other Long-Term Investments | Level 2 |  |
Investment securities |  |
Other long-term investments | 26.2 | 14.8
Fair value measured on a recurring basis | Other Long-Term Investments | Level 3 |  |
Investment securities |  |
Other long-term investments | 1,138.2 | 911.6
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 309.8 | 291.2
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 309.8 | 291.2
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 214.6 | 191.9
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 214.6 | 191.9
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 121.6 | 115.4
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 121.6 | 115.4
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 121.3 | 121.3
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 121.3 | 121.3
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 108.9 | 123.7
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 108.9 | 123.7
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 70.2 | 73.8
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 70.2 | 73.8
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 46.3 | 47.9
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 46.3 | 47.9
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 28.9 | 33.9
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 28.9 | 33.9
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations |  |
Investment securities |  |
Fixed maturity investments, at fair value | 23.4 | 19.7
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 0.0 | 0.0
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2 |  |
Investment securities |  |
Fixed maturity investments, at fair value | 23.4 | 19.7
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3 |  |
Investment securities |  |
Fixed maturity investments, at fair value | $ 0.0 | $ 0.0

X
- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)(b)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1A -Publisher FASB -URI https://asc.fasb.org//1943274/2147482736/825-10-45-1A + Details
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- Definition Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482106/820-10-50-2 + Details
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