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Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions | Sep. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | --- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | $ 1,622.7 | $ 1,377.8 |  |
Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 974.5 | 992.9 |  |
Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 240.7 | 277.6 |  |
Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 157.3 | 135.0 |  |
Level 2 | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 240.7 | 277.6 |  |
Level 2 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 157.3 | 135.0 |  |
Level 3 | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,044.1 | 890.6 | $ 814.5 | $ 614.2
Fair value measured on a recurring basis |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 5,154.4 | 4,265.3 |  |
Other long-term investments | 1,622.7 | 1,377.8 |  |
Fair value measured on a recurring basis | Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 148.2 | 261.6 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 216.9 | 211.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 974.5 | 992.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 269.0 | 264.2 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 181.4 | 178.1 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 115.8 | 112.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Healthcare |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 114.3 | 112.8 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 110.8 | 117.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 75.9 | 70.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 49.1 | 56.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 36.1 | 48.0 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 22.1 | 32.1 |  |
Fair value measured on a recurring basis | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 264.9 | 291.9 |  |
Fair value measured on a recurring basis | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 240.7 | 277.6 |  |
Fair value measured on a recurring basis | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 157.3 | 135.0 |  |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,854.3 | 1,908.9 |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Short-term investments, at fair value | 1,196.4 | 465.9 |  |
Fair value measured on a recurring basis | Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 332.8 | 251.1 |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 1,058.0 | 895.3 |  |
Fair value measured on a recurring basis | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 564.7 | 482.5 |  |
Fair value measured on a recurring basis | Level 1 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,607.9 | 939.0 |  |
Other long-term investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 148.2 | 261.6 |  |
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 216.9 | 211.5 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Healthcare |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Utilities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations | Materials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 216.9 | 211.5 |  |
Fair value measured on a recurring basis | Level 1 | Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Short-term investments, at fair value | 1,196.4 | 465.9 |  |
Fair value measured on a recurring basis | Level 1 | Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 46.4 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 1 | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 2 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,937.7 | 1,953.2 |  |
Other long-term investments | 13.9 | 4.7 |  |
Fair value measured on a recurring basis | Level 2 | Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 2 | U.S. Government and agency obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 974.5 | 992.9 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 269.0 | 264.2 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 181.4 | 178.1 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 115.8 | 112.9 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Healthcare |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 114.3 | 112.8 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 110.8 | 117.9 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 75.9 | 70.9 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 49.1 | 56.0 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 36.1 | 48.0 |  |
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 22.1 | 32.1 |  |
Fair value measured on a recurring basis | Level 2 | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 264.9 | 291.9 |  |
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 240.7 | 277.6 |  |
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 157.3 | 135.0 |  |
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,637.4 | 1,697.4 |  |
Fair value measured on a recurring basis | Level 2 | Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Short-term investments, at fair value | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 286.4 | 251.1 |  |
Fair value measured on a recurring basis | Level 2 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 13.9 | 4.7 |  |
Fair value measured on a recurring basis | Level 2 | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,044.1 | 890.6 |  |
Other long-term investments | 1,044.1 | 890.6 |  |
Fair value measured on a recurring basis | Level 3 | Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | U.S. Government and agency obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Healthcare |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Utilities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Debt securities issued by corporations | Materials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Short-term investments, at fair value | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Common equity securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 1,044.1 | 890.6 |  |
Fair value measured on a recurring basis | Level 3 | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | $ 0.0 | $ 0.0 |  |

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