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Investment Securities - Fair Value Measurement by Level (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	$ 637.0	$ 668.4
Other long-term investments	1,895.8	1,488.0
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	961.6	1,018.8
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	238.2	258.6
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,977.9	1,920.9
Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Level 3 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	0.0	0.0
Level 3 | Collateralized loan obligations
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 and Equity Securities, FV-NI [Line Items]
Fair value investments	5,862.4	5,170.0
Fair value measured on a recurring basis | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	188.8	168.6
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	261.1	206.4
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	961.6	1,018.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	290.1	291.2
Fair value measured on a recurring basis | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	188.3	191.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	109.1	121.3
Fair value measured on a recurring basis | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.8	115.4
Fair value measured on a recurring basis | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.5	123.7
Fair value measured on a recurring basis | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	66.6	73.8
Fair value measured on a recurring basis | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	42.4	47.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.1	33.9
Fair value measured on a recurring basis | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	20.7	19.7
Fair value measured on a recurring basis | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	238.2	258.6
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	325.4	254.2
Fair value measured on a recurring basis | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Fair value measured on a recurring basis | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,977.9	1,920.9
Fair value measured on a recurring basis | Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Short-term investments, at fair value	1,162.9	924.1
Fair value measured on a recurring basis | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	252.1	333.8
Fair value measured on a recurring basis | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	384.9	334.6
Fair value measured on a recurring basis | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	637.0	668.4
Fair value measured on a recurring basis | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,895.8	1,488.0
Fair value measured on a recurring basis | Fair Value, Inputs, Level 1, Level 2, and Level 3 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,090.7	926.4
Fair value measured on a recurring basis | Level 1
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,851.7	1,632.9
Fair value measured on a recurring basis | Level 1 | MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	188.8	168.6
Fair value measured on a recurring basis | Level 1 | U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
Fair value measured on a recurring basis | Level 1 | Debt securities issued by corporations
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 | Level 1 | Debt securities issued by corporations | Financials
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 | Level 1 | Debt securities issued by corporations | Consumer
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 | Level 1 | Debt securities issued by corporations | Healthcare
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 | Level 1 | Debt securities issued by corporations | Industrial
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 | Level 1 | Debt securities issued by corporations | Technology
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 | Level 1 | Debt securities issued by corporations | Utilities
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 | Level 1 | Debt securities issued by corporations | Communications
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 | Level 1 | Debt securities issued by corporations | Energy
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 | Level 1 | Debt securities issued by corporations | Materials
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 | Level 1 | Municipal obligations
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 | Level 1 | Mortgage and asset-backed securities
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 | Level 1 | Collateralized loan obligations
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 | Level 1 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	261.1	206.4
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	1,149.7	924.1
Fair value measured on a recurring basis | Level 1 | Exchange-traded funds
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	252.1	333.8
Fair value measured on a recurring basis | Level 1 | Common stock, other
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 | Level 1 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	252.1	333.8
Fair value measured on a recurring basis | Level 1 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	0.0	0.0
Fair value measured on a recurring basis | Level 1 | Other Long-Term Investments
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
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	2,130.3	2,063.9
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 2 | U.S. Government and agency obligations
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 | Level 2 | Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	961.6	1,018.8
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Financials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	290.1	291.2
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Consumer
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	188.3	191.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Healthcare
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	109.1	121.3
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Industrial
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.8	115.4
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Technology
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	107.5	123.7
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Utilities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	66.6	73.8
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Communications
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	42.4	47.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Energy
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	29.1	33.9
Fair value measured on a recurring basis | Level 2 | Debt securities issued by corporations | Materials
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	20.7	19.7
Fair value measured on a recurring basis | Level 2 | Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	238.2	258.6
Fair value measured on a recurring basis | Level 2 | Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	325.4	254.2
Fair value measured on a recurring basis | Level 2 | Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	191.6	182.9
Fair value measured on a recurring basis | Level 2 | Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Fixed maturity investments, at fair value	1,716.8	1,714.5
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	13.2	0.0
Fair value measured on a recurring basis | Level 2 | Exchange-traded funds
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 | Level 2 | Common stock, other
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	384.9	334.6
Fair value measured on a recurring basis | Level 2 | Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Common equity securities, at fair value	384.9	334.6
Fair value measured on a recurring basis | Level 2 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.4	14.8
Fair value measured on a recurring basis | Level 2 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	15.4	14.8
Fair value measured on a recurring basis | Level 3
Debt and Equity Securities, FV-NI [Line Items]
Fair value investments	1,075.3	911.6
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 | Level 3 | U.S. Government and agency obligations
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 | Level 3 | Debt securities issued by corporations
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 | Level 3 | Debt securities issued by corporations | Financials
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 | Level 3 | Debt securities issued by corporations | Consumer
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 | Level 3 | Debt securities issued by corporations | Healthcare
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 | Level 3 | Debt securities issued by corporations | Industrial
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 | Level 3 | Debt securities issued by corporations | Technology
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 | Level 3 | Debt securities issued by corporations | Utilities
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 | Level 3 | Debt securities issued by corporations | Communications
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 | Level 3 | Debt securities issued by corporations | Energy
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 | Level 3 | Debt securities issued by corporations | Materials
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 | Level 3 | Municipal obligations
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 | Level 3 | Mortgage and asset-backed securities
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 | Level 3 | Collateralized loan obligations
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 | Level 3 | Fixed maturity investments
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 | 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 | Exchange-traded funds
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 | Level 3 | Common stock, other
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 | Level 3 | Common equity securities
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 | Level 3 | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,075.3	911.6
Fair value measured on a recurring basis | Level 3 | Other Long-Term Investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	1,075.3	911.6
Fair value measured on a recurring basis | NAV | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Other long-term investments	$ 805.1	$ 561.6

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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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