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Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | --- | ---
Investment securities |  |  |
Other long-term investments | $ 1,488.0 | $ 1,377.8 |
Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 1,018.8 | 992.9 |
Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 254.2 | 277.6 |
Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 182.9 | 135.0 |
Level 2 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 254.2 | 277.6 |
Level 2 | Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 182.9 | 135.0 |
Level 3 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Level 3 | Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Level 3 | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 911.6 | 890.6 | $ 614.2
Fair value measured on a recurring basis |  |  |
Investment securities |  |  |
Fair value investments | 5,170.0 | 4,265.3 |
Other long-term investments | 1,488.0 | 1,377.8 |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 206.4 | 211.5 |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 1,018.8 | 992.9 |
Fair value measured on a recurring basis | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 258.6 | 291.9 |
Fair value measured on a recurring basis | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 254.2 | 277.6 |
Fair value measured on a recurring basis | Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 182.9 | 135.0 |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 1,920.9 | 1,908.9 |
Fair value measured on a recurring basis | Short-term investments |  |  |
Investment securities |  |  |
Short-term investments, at fair value | 924.1 | 465.9 |
Fair value measured on a recurring basis | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 668.4 | 251.1 |
Fair value measured on a recurring basis | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 926.4 | 895.3 |
Fair value measured on a recurring basis | Other long-term investments — NAV |  |  |
Investment securities |  |  |
Fair value investments | 561.6 | 482.5 |
Fair value measured on a recurring basis | MediaAlpha |  |  |
Investment securities |  |  |
Other long-term investments | 168.6 | 261.6 |
Fair value measured on a recurring basis | Level 1 |  |  |
Investment securities |  |  |
Fair value investments | 1,632.9 | 939.0 |
Other long-term investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 206.4 | 211.5 |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 206.4 | 211.5 |
Fair value measured on a recurring basis | Level 1 | Short-term investments |  |  |
Investment securities |  |  |
Short-term investments, at fair value | 924.1 | 465.9 |
Fair value measured on a recurring basis | Level 1 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 333.8 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | Other long-term investments — NAV |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 1 | MediaAlpha |  |  |
Investment securities |  |  |
Other long-term investments | 168.6 | 261.6 |
Fair value measured on a recurring basis | Level 2 |  |  |
Investment securities |  |  |
Fair value investments | 2,063.9 | 1,953.2 |
Other long-term investments | 14.8 | 4.7 |
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 1,018.8 | 992.9 |
Fair value measured on a recurring basis | Level 2 | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 258.6 | 291.9 |
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 254.2 | 277.6 |
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 182.9 | 135.0 |
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 1,714.5 | 1,697.4 |
Fair value measured on a recurring basis | Level 2 | Short-term investments |  |  |
Investment securities |  |  |
Short-term investments, at fair value | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 2 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 334.6 | 251.1 |
Fair value measured on a recurring basis | Level 2 | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 14.8 | 4.7 |
Fair value measured on a recurring basis | Level 2 | Other long-term investments — NAV |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
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 | 911.6 | 890.6 |
Other long-term investments | 911.6 | 890.6 |
Fair value measured on a recurring basis | Level 3 | U.S. Government and agency obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Municipal obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Mortgage and asset-backed securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Collateralized loan obligations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Short-term investments |  |  |
Investment securities |  |  |
Short-term investments, at fair value | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | Other long-term investments |  |  |
Investment securities |  |  |
Fair value investments | 911.6 | 890.6 |
Fair value measured on a recurring basis | Level 3 | Other long-term investments — NAV |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Fair value measured on a recurring basis | Level 3 | MediaAlpha |  |  |
Investment securities |  |  |
Other long-term investments | 0.0 | 0.0 |
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 291.2 | 264.2 |
Financials | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Financials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 291.2 | 264.2 |
Financials | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 191.9 | 178.1 |
Consumer | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Consumer | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 191.9 | 178.1 |
Consumer | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 123.7 | 117.9 |
Technology | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Technology | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 123.7 | 117.9 |
Technology | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 121.3 | 112.8 |
Healthcare | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Healthcare | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 121.3 | 112.8 |
Healthcare | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 115.4 | 112.9 |
Industrial | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Industrial | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 115.4 | 112.9 |
Industrial | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 73.8 | 70.9 |
Utilities | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Utilities | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 73.8 | 70.9 |
Utilities | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 47.9 | 56.0 |
Communications | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Communications | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 47.9 | 56.0 |
Communications | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 33.9 | 48.0 |
Energy | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Energy | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 33.9 | 48.0 |
Energy | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 19.7 | 32.1 |
Materials | Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | 0.0 |
Materials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 19.7 | 32.1 |
Materials | Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |
Investment securities |  |  |
Fair value investments | 0.0 | $ 0.0 |
Exchange traded funds | Fair value measured on a recurring basis | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 333.8 |  |
Exchange traded funds | Fair value measured on a recurring basis | Level 1 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 333.8 |  |
Exchange traded funds | Fair value measured on a recurring basis | Level 2 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Exchange traded funds | Fair value measured on a recurring basis | Level 3 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Other | Fair value measured on a recurring basis | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 334.6 |  |
Other | Fair value measured on a recurring basis | Level 1 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 0.0 |  |
Other | Fair value measured on a recurring basis | Level 2 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | 334.6 |  |
Other | Fair value measured on a recurring basis | Level 3 | Total common equity securities |  |  |
Investment securities |  |  |
Fair value investments | $ 0.0 |  |

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