Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	$ 223.8	$ (58.3)	$ 390.4	$ 151.2
MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	88.2	(46.8)	159.7	(38.9)
Net realized and unrealized investment (losses) gains
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	4.3	1.3	119.2	60.7
Net unrealized investment gains (losses)	219.5	(59.6)	271.2	90.5
Fixed maturity investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	(0.5)	(0.3)	(9.2)	(1.7)
Net unrealized investment gains (losses)	56.9	(27.9)	49.7	(9.7)
Short-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0.0	(0.2)	(0.3)	(0.3)
Net unrealized investment gains (losses)	0.5	(0.6)	(0.5)	0.7
Common equity securities
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0.0	1.2	3.5	13.8
Net unrealized investment gains (losses)	24.7	(18.5)	63.6	29.8
Net realized and unrealized investment gains (losses) (1)	24.7	(17.3)	67.1	43.6
Equity Method Investments \| MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	0.0	0.0	91.2	0.0
Net unrealized investment gains (losses)	88.2	(46.8)	68.5	(38.9)
Net realized and unrealized investment gains (losses) (1)	88.2	(46.8)	159.7	(38.9)
Other long-term investments
Gain (Loss) on Securities [Line Items]
Net realized investment gains (losses)	4.8	0.6	34.0	48.9
Net unrealized investment gains (losses)	49.2	34.2	89.9	108.6
Fixed maturity and short-term investments
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	56.9	(29.0)	39.7	(11.0)
Less: net realized and unrealized gains (losses) on investment securities sold during the period	1.2	2.5	0.8	4.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	55.7	(31.5)	38.9	(15.7)
Common equity securities and investment in MediaAlpha
Gain (Loss) on Securities [Line Items]
Net realized and unrealized investment gains (losses) (1)	112.9	(64.1)	226.8	4.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period	0.0	(0.1)	35.4	3.7
Net unrealized investment gains (losses) recognized during the period on investment securities held at the end of the period	112.9	(64.0)	191.4	1.0
Total Investments After TPC Providers - Investments
Gain (Loss) on Securities [Line Items]
Gain (loss) on investments, exchange rate effect	$ 18.9	$ (12.8)	$ 7.1	$ (1.9)

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

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