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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

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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