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Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	$ (40.6)	$ (156.7)
Less: net losses on investment securities sold during the period	(0.2)	(8.7)
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	(41.7)	30.0
Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	(40.4)	(148.0)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	(25.4)	2.2
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	(1.3)	0.4
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	1.0	(140.8)
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	(41.7)	30.0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	28.1	(48.5)
Gain (loss) on investments, exchange rate effect	(0.2)	(0.4)
Amount attributable to TPC Providers
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha	$ (1.3)	$ 0.0

X

- Definition

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

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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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

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