Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 220.2	$ (37.9)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(50.5)	6.3
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	169.7	(31.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.3)	(10.7)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.3)	(10.7)
Net unrealized investment gains (losses), pre-tax (1)	219.9	(48.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(50.5)	6.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	169.4	(42.3)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	22.0	(18.6)
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	(14.8)
Net unrealized investment gains (losses), pre-tax (1)	22.0	(33.4)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	0.0	(0.7)
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0
Net unrealized investment gains (losses), pre-tax (1)	0.0	(0.7)
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	85.5	(16.7)
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0
Net unrealized investment gains (losses), pre-tax (1)	85.5	(16.7)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	112.7	(1.9)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.3)	4.1
Net unrealized investment gains (losses), pre-tax (1)	112.4	$ 2.2
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain	$ 114.7