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Investment Securities (Fair Value Measurement by Level) (Details) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2019 USD ($) Investment | Dec. 31, 2018 USD ($) Investment | Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($)
Investment securities |  |  |  |
Percentage of investments recorded at fair value | 71.00% | 87.00% |  |
Fair value investments | $ 2,746.0 | $ 2,328.7 |  | $ 3,208.3
Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 467.2 | 510.5 |  |
Mortgage and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | $ 209.0 | $ 133.5 |  |
Common Stock |  |  |  |
Investment securities |  |  |  |
Number of investments | Investment | 2 | 2 |  |
Level 1 |  |  |  |
Investment securities |  |  |  |
Fair value investments | $ 780.0 | $ 842.6 | $ 842.6 | 890.4
Level 2 |  |  |  |
Investment securities |  |  |  |
Fair value investments | 1,109.6 | 1,160.5 | 1,160.5 | 2,105.4
Level 2 | Mortgage and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 209.0 | 133.5 |  |
Level 3 Inputs | Mortgage and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.1 | 0.0 |  |
Level 3 Inputs | Other long-term investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 654.0 | 138.7 | 138.7 | 77.2
Fair Value Measured at Net Asset Value Per Share | Hedge Funds and Private Equity Funds |  |  |  |
Investment securities |  |  |  |
Fair value investments | 202.3 | 186.9 |  | $ 135.3
Fair value measured on a recurring basis |  |  |  |
Investment securities |  |  |  |
Fair value investments | 2,947.2 | 2,542.9 |  |
Fair value measured on a recurring basis | U.S. Government and agency obligations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 232.5 | 153.2 |  |
Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 467.2 | 510.5 |  |
Fair value measured on a recurring basis | Mortgage and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 209.0 | 133.5 |  |
Fair value measured on a recurring basis | Municipal obligations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 297.1 | 280.3 |  |
Fair value measured on a recurring basis | Fixed maturity investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 1,205.8 | 1,077.5 |  |
Fair value measured on a recurring basis | Short-term investments |  |  |  |
Investment securities |  |  |  |
Short-term investments, at fair value | 201.2 | 214.2 |  |
Fair value measured on a recurring basis | Other long-term investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 654.0 | 138.7 |  |
Fair value measured on a recurring basis | Level 1 |  |  |  |
Investment securities |  |  |  |
Fair value investments | 969.4 | 1,047.0 |  |
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 232.5 | 153.2 |  |
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 | Mortgage and asset-backed securities |  |  |  |
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 | Fixed maturity investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 232.5 | 153.2 |  |
Fair value measured on a recurring basis | Level 1 | Short-term investments |  |  |  |
Investment securities |  |  |  |
Short-term investments, at fair value | 189.4 | 204.4 |  |
Fair value measured on a recurring basis | Level 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 547.5 |  |  |
Fair value measured on a recurring basis | Level 1 | Other long-term investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 |  | $ 2,328.7 |
Fair value measured on a recurring basis | Level 2 |  |  |  |
Investment securities |  |  |  |
Fair value investments | 1,121.4 | 1,170.3 |  |
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 | 467.2 | 510.5 |  |
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 209.0 | 133.5 |  |
Fair value measured on a recurring basis | Level 2 | Municipal obligations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 297.1 | 280.3 |  |
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 973.3 | 924.3 |  |
Fair value measured on a recurring basis | Level 2 | Short-term investments |  |  |  |
Investment securities |  |  |  |
Short-term investments, at fair value | 11.8 | 9.8 |  |
Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 136.3 |  |  |
Fair value measured on a recurring basis | Level 2 | Other long-term investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs |  |  |  |
Investment securities |  |  |  |
Fair value investments | 654.1 | 138.7 |  |
Fair value measured on a recurring basis | Level 3 Inputs | U.S. Government and agency obligations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs | Mortgage and asset-backed securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs | Municipal obligations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs | Fixed maturity investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs | Short-term investments |  |  |  |
Investment securities |  |  |  |
Short-term investments, at fair value | 0.0 | 0.0 |  |
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments |  |  |  |
Investment securities |  |  |  |
Fair value investments | 654.0 | 138.7 |  |
Fair value measured on a recurring basis | Fair Value Measured at Net Asset Value Per Share | Other long-term investments |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 186.9 |  |
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 144.8 | 143.4 |  |
Financials | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 13.5 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 13.5 |  |
Financials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 144.8 | 143.4 |  |
Financials | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Financials | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Financials | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 59.0 | 47.6 |  |
Industrial | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 11.4 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 11.4 |  |
Industrial | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 59.0 | 47.6 |  |
Industrial | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Industrial | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Industrial | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 52.6 | 55.0 |  |
Healthcare | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 14.0 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 14.0 |  |
Healthcare | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 52.6 | 55.0 |  |
Healthcare | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Healthcare | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Healthcare | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 50.9 | 68.5 |  |
Consumer | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 6.2 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 6.2 |  |
Consumer | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 50.9 | 68.5 |  |
Consumer | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Consumer | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Consumer | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 44.9 | 57.6 |  |
Energy | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 4.1 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 4.1 |  |
Energy | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 44.9 | 57.6 |  |
Energy | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Energy | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Energy | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 41.2 | 60.5 |  |
Technology | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 7.4 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 7.4 |  |
Technology | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 41.2 | 60.5 |  |
Technology | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Technology | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Technology | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 31.3 | 31.8 |  |
Communications | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 12.7 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 12.7 |  |
Communications | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 31.3 | 31.8 |  |
Communications | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Communications | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Communications | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 25.0 | 19.8 |  |
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 | 25.0 | 19.8 |  |
Utilities | Fair value measured on a recurring basis | Level 3 Inputs | 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 | 17.5 | 26.3 |  |
Materials | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 3.1 |  |
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 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 3.1 |  |
Materials | Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 17.5 | 26.3 |  |
Materials | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Materials | Fair value measured on a recurring basis | Level 3 Inputs | Debt securities issued by corporations |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Materials | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 0.0 |  |
Exchange traded funds | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 536.4 | 675.3 |  |
Exchange traded funds | Fair value measured on a recurring basis | Level 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 521.6 | 617.0 |  |
Exchange traded funds | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 14.8 | 58.3 |  |
Exchange traded funds | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.0 | 0.0 |  |
Other | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 147.5 | 177.9 |  |
Other | Fair value measured on a recurring basis | Level 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 25.9 | 0.0 |  |
Other | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 121.5 | 177.9 |  |
Other | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 0.1 | 0.0 |  |
Common equity securities | Fair value measured on a recurring basis | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | 683.9 | 925.6 |  |
Common equity securities | Fair value measured on a recurring basis | Level 1 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 689.4 |  |
Common equity securities | Fair value measured on a recurring basis | Level 2 | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments |  | 236.2 |  |
Common equity securities | Fair value measured on a recurring basis | Level 3 Inputs | Common equity securities |  |  |  |
Investment securities |  |  |  |
Fair value investments | $ 0.1 | $ 0.0 |  |

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