Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	$ 483.0	$ 650.0
Other long-term investments	2,958.5	2,150.2
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	460.3	462.1
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,537.7	1,414.2
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value		3.2
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	393.0	373.9
Mortgage and asset-backed securities | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	393.0	373.9
Mortgage and asset-backed securities | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	347.0	236.7
Collateralized loan obligations | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	347.0	236.7
Collateralized loan obligations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Foreign government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	32.5	21.5
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,770.5	2,511.6
Fair value measured on a recurring basis
Debt and Equity Securities, FV-NI [Line Items]
Total investments	8,324.9	6,477.6
Fair value measured on a recurring basis | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	231.2	201.6
Fair value measured on a recurring basis | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Total investments	2,593.9	1,838.6
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	231.2	201.6
Fair value measured on a recurring basis | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Total investments	2,806.2	2,512.3
Fair value measured on a recurring basis | Level 2 | 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
Debt and Equity Securities, FV-NI [Line Items]
Total investments	1,814.4	1,262.7
Fair value measured on a recurring basis | Level 3 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	460.3	462.1
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	460.3	461.3
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.8
Fair value measured on a recurring basis | U.S. Government and agency obligations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,537.7	1,414.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,537.7	1,414.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value		3.2
Fair value measured on a recurring basis | Municipal obligations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value		0.0
Fair value measured on a recurring basis | Municipal obligations | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value		3.2
Fair value measured on a recurring basis | Municipal obligations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value		0.0
Fair value measured on a recurring basis | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	393.0	373.9
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	393.0	373.9
Fair value measured on a recurring basis | Mortgage and asset-backed securities | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	347.0	236.7
Fair value measured on a recurring basis | Collateralized loan obligations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Collateralized loan obligations | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	347.0	236.7
Fair value measured on a recurring basis | Collateralized loan obligations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Foreign government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	32.5	21.5
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	32.5	21.5
Fair value measured on a recurring basis | Foreign government and agency obligations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,770.5	2,511.6
Fair value measured on a recurring basis | Fixed maturity investments | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	460.3	461.3
Fair value measured on a recurring basis | Fixed maturity investments | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	2,310.2	2,050.3
Fair value measured on a recurring basis | Fixed maturity investments | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	1,881.7	964.2
Fair value measured on a recurring basis | Short-term investments | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	1,871.7	951.1
Fair value measured on a recurring basis | Short-term investments | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	10.0	13.1
Fair value measured on a recurring basis | Short-term investments | Level 3
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 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	30.7	224.6
Fair value measured on a recurring basis | Exchange-traded funds | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	30.7	224.6
Fair value measured on a recurring basis | Exchange-traded funds | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0.0	0.0
Fair value measured on a recurring basis | Exchange-traded funds | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0.0	0.0
Fair value measured on a recurring basis | Other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	452.3	425.4
Fair value measured on a recurring basis | Other | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0.0	0.0
Fair value measured on a recurring basis | Other | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	452.3	425.4
Fair value measured on a recurring basis | Other | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	0.0	0.0
Fair value measured on a recurring basis | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	483.0	650.0
Fair value measured on a recurring basis | Common equity securities | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	30.7	224.6
Fair value measured on a recurring basis | Common equity securities | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	452.3	425.4
Fair value measured on a recurring basis | Common equity securities | Level 3
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,848.1	1,286.2
Fair value measured on a recurring basis | Other long-term investments | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Other long-term investments | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	33.7	23.5
Fair value measured on a recurring basis | Other long-term investments | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,814.4	1,262.7
Fair value measured on a recurring basis | Other long-term investments | NAV
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,110.4	864.0
Fair value measured on a recurring basis | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	2,958.5	2,150.2
Fair value measured on a recurring basis | Other Long-Term Investments | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Other Long-Term Investments | Level 2
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	33.7	23.5
Fair value measured on a recurring basis | Other Long-Term Investments | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,814.4	1,262.7
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	531.5	466.7
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	531.5	466.7
Financials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.2	255.2
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	306.2	255.2
Consumer | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	150.0	164.4
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	150.0	164.4
Industrial | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	146.1	153.1
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	146.1	153.1
Healthcare | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	98.4	113.8
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	98.4	113.8
Technology | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	90.4	60.9
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	90.4	60.9
Utilities | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	78.7	65.7
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	78.7	65.7
Materials | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	76.3	71.6
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	76.3	71.6
Communications | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	60.1	62.8
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 1
Debt and Equity Securities, FV-NI [Line Items]
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
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	60.1	62.8
Energy | Fair value measured on a recurring basis | Debt securities issued by corporations | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	$ 0.0	$ 0.0