Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	$ 156.8	$ 187.1	$ 96.6
Unrealized investment gains (losses)	(395.2)	529.7	336.6
Gain (loss) on investments related to foreign currency exchange	(7.7)	4.0	(0.3)
Gross realized investment gains	212.3	214.4	104.0
Gross realized investment losses	55.5	27.3	7.4
Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(246.1)	716.8	433.2
Fixed maturity investments
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	3.9	10.9	4.9
Unrealized investment gains (losses)	(42.2)	27.5	38.6
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	(0.1)	0.4	0.2
Common equity securities
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	0.4	137.2	85.9
Unrealized investment gains (losses)	14.9	(130.6)	109.8
Net realized and unrealized investment gains (losses)	15.3	6.6	195.7
MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	160.3	63.8	0.0
Unrealized investment gains (losses)	(540.6)	622.2	180.0
Net realized and unrealized investment gains (losses)	(380.3)	686.0	180.0
Other long-term investments
Debt Securities, Available-for-sale [Line Items]
Realized investment gains (losses)	(7.7)	(25.2)	5.6
Unrealized investment gains (losses)	172.7	10.6	8.2
Total Investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(238.4)	716.8	433.2
Amount attributable to TPC Providers
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(7.7)	0.0	0.0
Fixed maturity and short-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(38.4)	38.8	43.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period	(8.4)	(8.7)	1.0
Fixed maturity and short-term investments \| Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(30.0)	47.5	42.7
Common equity securities and investment in MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(365.0)	692.6	375.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period	20.3	38.3	24.0
Common equity securities and investment in MediaAlpha \| Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	$ (385.3)	$ 654.3	$ 351.7

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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