Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	$ 184.9	$ 434.2	$ (191.5)
Amount attributable to TPC Providers	0.0	0.0	6.8
Investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	38.0	27.1	(93.0)
Net realized and unrealized investment (losses) gains
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	118.0	96.8	115.1
Net unrealized investment gains (losses)	66.9	337.4	(313.4)
Net realized and unrealized investment gains (losses)	184.9	434.2	(198.3)
Fixed maturity investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(10.1)	(4.7)	(11.9)
Net unrealized investment gains (losses)	3.9	66.1	(168.4)
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(1.2)	(0.2)	(1.4)
Net unrealized investment gains (losses)	(0.8)	1.9	(1.1)
Common equity securities
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	3.5	29.5	0.0
Net unrealized investment gains (losses)	58.1	47.1	(7.1)
Net realized and unrealized investment gains (losses)	61.6	76.6	(7.1)
Equity Method Investments \| Investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	91.2	0.0	0.0
Net unrealized investment gains (losses)	(53.2)	27.1	(93.0)
Net realized and unrealized investment gains (losses)	38.0	27.1	(93.0)
Other long-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	34.6	72.2	128.4
Net unrealized investment gains (losses)	58.9	195.2	(43.8)
Fixed Maturities And Other Short-Term Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(8.2)	63.1	(182.8)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	0.9	4.7	(2.9)
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	(9.1)	58.4	(179.9)
Common Stock And Equity Method Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	99.6	103.7	(100.1)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	35.4	28.4	0.0
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	64.2	75.3	(100.1)
Total Investments After TPC Providers - Investments
Debt Securities, Available-for-sale [Line Items]
Gain (loss) on investments, exchange rate effect	$ (31.5)	$ 13.6	$ (29.3)

Amount, after tax, of realized and unrealized gain (loss) from foreign currency transaction.

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