Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	$ 12.6	$ (2.9)	$ 59.4	$ 14.5
Net unrealized investment gains (losses)	(1.2)	(217.4)	150.1	(226.5)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(77.3)	(113.5)	7.9	(94.7)
Gain (loss) on investments, exchange rate effect	5.5	(24.5)	10.9	(21.8)
Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment gains (losses)	11.4	(217.6)	209.5	(207.2)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	(1.3)	(4.8)	(1.4)	(5.9)
Net unrealized investment gains (losses)	(11.5)	(54.6)	18.2	(134.1)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	0.2	(0.1)	(0.1)	(0.3)
Net unrealized investment gains (losses)	0.2	(0.7)	1.3	(0.5)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	12.6	0.0	12.6	0.0
Net unrealized investment gains (losses)	18.9	(11.6)	48.3	(10.6)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	31.5	(11.6)	60.9	(10.6)
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses)	(77.3)	(113.5)	7.9	(94.7)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(77.3)	(113.5)	7.9	(94.7)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	1.1	2.0	48.3	20.7
Net unrealized investment gains (losses)	68.5	(37.0)	74.4	13.4
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	11.4	(220.3)	209.5	(212.0)
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	2.7	0.0	4.8
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	(12.4)	(60.2)	18.0	(140.8)
Net realized and unrealized investment gains (losses)	0.4	(2.1)	2.2	(3.1)
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	(12.8)	(58.1)	15.8	(137.7)
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	(45.8)	(125.1)	68.8	(105.3)
Net realized and unrealized investment gains (losses)	3.8	0.0	3.8	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	$ (49.6)	$ (125.1)	$ 65.0	$ (105.3)

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

Reference 1: http://www.xbrl.org/2003/role/recommendedDisclosureRef
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c),9(a))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

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The net realized gain (loss) on investments sold during the period, not including gains (losses) on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, which, for cash flow reporting, is a component of proceeds from investing activities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147482740/230-10-45-28

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(3)(a))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1

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Amount of unrealized gain (loss) on investment.

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Gain (Loss) On Investments, Exchange Rate Effect

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