Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	$ 716.8	$ 433.2	$ (108.3)
Less: net gains (losses) on investment securities sold during the period	20.2	24.6	(33.0)
Realized gains	214.4	104.0	49.4
Realized losses	27.3	7.4	62.3
MediaAlpha
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	686.0	180.0	0.0
Investments Held
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	696.6	408.6	(75.3)
Fixed maturity investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	38.5	43.5	(34.7)
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	0.4	0.2	(0.8)
Common equity securities
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	6.5	195.7	(98.9)
Other long-term investments
Debt Securities, Available-for-sale [Line Items]
Net realized and unrealized investment gains (losses)	(14.6)	13.8	26.1
Gain (loss) on investments related to foreign currency exchange	$ 4.0	$ (0.3)	$ (0.8)

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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