Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Investment securities
Common equity securities, at fair value	$ 650.0	$ 538.4
Other long-term investments	2,150.2	1,998.2
U.S. Government and agency obligations
Investment securities
Fixed maturity investments, at fair value	462.1	204.9
Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	1,414.2	1,045.0
Municipal obligations
Investment securities
Fixed maturity investments, at fair value	3.2	260.9
Mortgage and asset-backed securities
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
Mortgage and asset-backed securities | Level 2
Investment securities
Fixed maturity investments, at fair value	373.9	389.4
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	236.7	209.1
Collateralized loan obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	236.7	209.1
Collateralized loan obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0.0	0.0
Foreign government and agency obligations
Investment securities
Fixed maturity investments, at fair value	21.5
Fixed maturity investments
Investment securities
Fixed maturity investments, at fair value	2,511.6	2,109.3
Fair value measured on a recurring basis
Investment securities
Fair value investments	6,477.6	6,388.7
Fair value measured on a recurring basis | MediaAlpha
Investment securities
Other long-term investments	201.6	254.9
Fair value measured on a recurring basis | Level 1
Investment securities
Fair value investments	1,838.6	2,055.2
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Investment securities
Other long-term investments	201.6	254.9
Fair value measured on a recurring basis | Level 2
Investment securities
Fair value investments	2,512.3	2,361.5
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	1,262.7	1,138.2
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	462.1	204.9
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1
Investment securities
Fixed maturity investments, at fair value	461.3	204.9
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	0.8	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	1,414.2	1,045.0
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	1,414.2	1,045.0
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	3.2	260.9
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	3.2	260.9
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	373.9	389.4
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	373.9	389.4
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	236.7	209.1
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	236.7	209.1
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 | Foreign government and agency obligations
Investment securities
Fixed maturity investments, at fair value	21.5
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 1
Investment securities
Fixed maturity investments, at fair value	0.0
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 2
Investment securities
Fixed maturity investments, at fair value	21.5
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 3
Investment securities
Fixed maturity investments, at fair value	0.0
Fair value measured on a recurring basis | Fixed maturity investments
Investment securities
Fixed maturity investments, at fair value	2,511.6	2,109.3
Fair value measured on a recurring basis | Fixed maturity investments | Level 1
Investment securities
Fixed maturity investments, at fair value	461.3	204.9
Fair value measured on a recurring basis | Fixed maturity investments | Level 2
Investment securities
Fixed maturity investments, at fair value	2,050.3	1,904.4
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	964.2	1,487.9
Fair value measured on a recurring basis | Short-term investments | Level 1
Investment securities
Short-term investments, at fair value	951.1	1,457.6
Fair value measured on a recurring basis | Short-term investments | Level 2
Investment securities
Short-term investments, at fair value	13.1	30.3
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	224.6	137.8
Fair value measured on a recurring basis | Exchange-traded funds | Level 1
Investment securities
Common equity securities, at fair value	224.6	137.8
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	425.4	400.6
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	425.4	400.6
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	650.0	538.4
Fair value measured on a recurring basis | Common equity securities | Level 1
Investment securities
Common equity securities, at fair value	224.6	137.8
Fair value measured on a recurring basis | Common equity securities | Level 2
Investment securities
Common equity securities, at fair value	425.4	400.6
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	1,286.2	1,164.4
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	23.5	26.2
Fair value measured on a recurring basis | Other long-term investments | Level 3
Investment securities
Other long-term investments	1,262.7	1,138.2
Fair value measured on a recurring basis | Other long-term investments | NAV
Investment securities
Other long-term investments	864.0	833.8
Fair value measured on a recurring basis | Other Long-Term Investments
Investment securities
Other long-term investments	2,150.2	1,998.2
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	23.5	26.2
Fair value measured on a recurring basis | Other Long-Term Investments | Level 3
Investment securities
Other long-term investments	1,262.7	1,138.2
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	466.7	309.8
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	466.7	309.8
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	255.2	214.6
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	255.2	214.6
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	164.4	121.6
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	164.4	121.6
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	153.1	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	153.1	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	113.8	108.9
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	113.8	108.9
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
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	71.6	46.3
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	71.6	46.3
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
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	65.7	23.4
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	65.7	23.4
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
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	62.8	28.9
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	62.8	28.9
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
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations
Investment securities
Fixed maturity investments, at fair value	60.9	70.2
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	60.9	70.2
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