Investment Securities - Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt and Equity Securities, FV-NI [Line Items]
Less: net (losses) gains on investment securities sold during the period	$ (4.8)	$ 36.1	$ (5.1)	$ 29.6
Investments Held
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	(361.5)	270.5	(229.2)	273.2
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	(6.8)	3.6	(23.2)	35.8
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	(0.3)	0.0	0.0	0.4
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	1.6	48.4	7.7	5.9
Marketing Technology (MediaAlpha)
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	(396.8)	250.0	(325.5)	295.0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	37.7	4.6	113.2	(34.3)
Gain (loss) on investments, exchange rate effect	(7.0)	0.8	(7.4)	(0.8)
Amount attributable to TPC Providers
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	(1.7)	0.0	(6.5)	0.0
Net realized and unrealized investment (losses) gains
Debt and Equity Securities, FV-NI [Line Items]
Net realized and unrealized investment (losses) gains	$ (366.3)	$ 306.6	$ (234.3)	$ 302.8