Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	$ 46.7	$ 17.4
Net unrealized investment gains (losses)	151.4	(9.1)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	85.2	18.8
Gain (loss) on investments, exchange rate effect	5.4	2.6
Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment gains (losses)	198.1	10.4
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	(0.1)	(1.1)
Net unrealized investment gains (losses)	29.7	(79.5)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	(0.4)	(0.2)
Net unrealized investment gains (losses)	1.2	0.2
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses)	29.4	1.0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	29.4	1.0
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses)	85.2	18.8
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	85.2	18.8
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	47.2	18.7
Net unrealized investment gains (losses)	5.9	50.4
Net realized and unrealized investment (losses) gains
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	198.1	8.3
TPC Providers
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	0.0	2.1
Fixed maturity and short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	30.4	(80.6)
Net realized and unrealized investment gains (losses)	1.8	(1.0)
Fixed maturity and short-term investments \| Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	28.6	(79.6)
Common equity securities and investment in MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	114.6	19.8
Net realized and unrealized investment gains (losses)	0.0	0.0
Common equity securities and investment in MediaAlpha \| Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	$ 114.6	$ 19.8

Amount of realized and unrealized gain (loss) on investment.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
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Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef
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