Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	$ 17.4	$ 143.0
Net unrealized investment gains (losses)	(9.1)	(182.3)
Gain (loss) on investments, exchange rate effect	2.6	(0.2)
Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment gains (losses)	10.4	(40.6)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	(1.1)	1.9
Net unrealized investment gains (losses)	(79.5)	(27.3)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	(0.2)	(0.3)
Net unrealized investment gains (losses)	0.2	(1.0)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses)	1.0	1.0
Net unrealized investment gains (losses) on investment securities held at the end of the period	1.0	1.0
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	0.0	160.3
Net unrealized investment gains (losses)	18.8	(202.0)
Net unrealized investment gains (losses) on investment securities held at the end of the period	18.8	(41.7)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized investment gains (losses)	18.7	(18.9)
Net unrealized investment gains (losses)	50.4	47.0
Net realized and unrealized investment (losses) gains
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) on investment securities held at the end of the period	8.3	(39.3)
TPC Providers
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) on investment securities held at the end of the period	2.1	(1.3)
Fixed maturity and short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) on investment securities held at the end of the period	(80.6)	(26.7)
Net realized and unrealized investment gains (losses)	(1.0)	(0.9)
Fixed maturity and short-term investments \| Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) on investment securities held at the end of the period	(79.6)	(25.8)
Common equity securities and investment in MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) on investment securities held at the end of the period	19.8	(40.7)
Net realized and unrealized investment gains (losses)	0.0	(19.9)
Common equity securities and investment in MediaAlpha \| Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net unrealized investment gains (losses) on investment securities held at the end of the period	$ 19.8	$ (60.6)

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

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