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Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | --- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | $ 3,232.6 | $ 2,796.2 | $ 2,541.6 | $ 2,746.0
Other long-term investments | 1,032.2 | 786.8 |  |
Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 686.9 | 547.4 |  |
Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 229.9 | 218.5 |  |
Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 59.4 | 0.0 |  |
Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Short-term investments, at fair value | 625.8 | 142.9 | 137.6 | 201.2
Level 1 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 821.8 | 978.5 | 645.6 | 780.0
Level 2 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,378.6 | 1,030.9 | 1,090.3 | 1,109.6
Level 2 | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 229.9 | 218.5 |  |
Level 2 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 59.4 | 0.0 |  |
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 | 656.4 | 614.2 | $ 660.9 | $ 654.0
Fair value measured on a recurring basis |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 3,858.4 | 2,939.1 |  |
Other long-term investments | 1,032.2 |  |  |
Fair value measured on a recurring basis | Marketing Technology (MediaAlpha) |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Other long-term investments | 600.2 | 802.2 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 221.6 | 176.3 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 686.9 | 547.4 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 183.1 | 133.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 96.3 | 81.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 74.2 | 66.9 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 81.9 | 66.7 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Healthcare |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 76.6 | 51.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 53.4 | 44.5 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 51.4 | 32.3 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 39.3 | 35.8 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 30.7 | 33.9 |  |
Fair value measured on a recurring basis | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 283.8 | 265.0 |  |
Fair value measured on a recurring basis | Mortgage and asset-backed securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 229.9 | 218.5 |  |
Fair value measured on a recurring basis | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 59.4 |  |  |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,481.6 | 1,207.2 |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Short-term investments, at fair value | 625.8 | 142.9 |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 786.8 |  |
Other long-term investments | 656.4 |  |  |
Fair value measured on a recurring basis | Common Equity Securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 118.6 |  |  |
Fair value measured on a recurring basis | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 172.6 |  |
Other long-term investments | 375.8 |  |  |
Fair value measured on a recurring basis | Other Long-term Investments, Excluding NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 614.2 |  |
Fair value measured on a recurring basis | Level 1 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,303.0 | 1,121.4 |  |
Other long-term investments | 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 | 600.2 | 802.2 |  |
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 | 221.6 | 176.3 |  |
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 | 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 | 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 | 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 | 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 | 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 |  |  |
Fair value measured on a recurring basis | Level 1 | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 221.6 | 176.3 |  |
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 | 481.2 | 142.9 |  |
Fair value measured on a recurring basis | Level 1 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Other long-term investments | 0.0 |  |  |
Fair value measured on a recurring basis | Level 1 | Common Equity Securities |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 0.0 |  |  |
Fair value measured on a recurring basis | Level 1 | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Other long-term investments | 0.0 |  |  |
Fair value measured on a recurring basis | Level 1 | Other Long-term Investments, Excluding NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Fair value measured on a recurring basis | Level 2 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,523.2 | 1,030.9 |  |
Other long-term investments | 0.0 |  |  |
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 | 686.9 | 547.4 |  |
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 | 183.1 | 133.9 |  |
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 | 96.3 | 81.9 |  |
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 | 74.2 | 66.9 |  |
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 | 81.9 | 66.7 |  |
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 | 76.6 | 51.5 |  |
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 | 53.4 | 44.5 |  |
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 | 51.4 | 32.3 |  |
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 | 39.3 | 35.8 |  |
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 | 30.7 | 33.9 |  |
Fair value measured on a recurring basis | Level 2 | Municipal obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 283.8 | 265.0 |  |
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 | 229.9 | 218.5 |  |
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 59.4 |  |  |
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 1,260.0 | 1,030.9 |  |
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 | 144.6 | 0.0 |  |
Fair value measured on a recurring basis | Level 2 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Other long-term investments | 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 | 118.6 |  |  |
Fair value measured on a recurring basis | Level 2 | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Other long-term investments | 0.0 |  |  |
Fair value measured on a recurring basis | Level 2 | Other Long-term Investments, Excluding NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Fair value measured on a recurring basis | Level 3 |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments | 656.4 | 614.2 |  |
Other long-term investments | 656.4 |  |  |
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 |  |  |
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 | 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 | 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 | 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 | 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 | 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 |  |  |
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 | Other long-term investments |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 614.2 |  |
Other long-term investments | 656.4 |  |  |
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 |  |  |
Fair value measured on a recurring basis | Level 3 | Other long-term investments — NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | 0.0 |  |
Other long-term investments | $ 0.0 |  |  |
Fair value measured on a recurring basis | Level 3 | Other Long-term Investments, Excluding NAV |  |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |  |
Fair value investments |  | $ 614.2 |  |

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