Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ (10.6)	$ 9.9	$ 209.6	$ (28.0)
Net Foreign Exchange Gains, pre-tax	0.3	(0.8)	0.0	(11.5)
Net unrealized investment gains (losses), pre-tax	(10.3)	9.1	209.6	(39.5)
Net Unrealized Gains (Losses), tax	(3.0)	11.4	(47.5)	5.1
Net Foreign Exchange Gains, tax	0.0	(0.1)	0.0	0.0
Total Net Unrealized Gains (Losses) Reflected in Earnings, tax	(3.0)	11.3	(47.5)	5.1
Net Unrealized Gains (Losses), after-tax	(7.6)	(1.5)	162.1	(33.1)
Net Foreign Exchange Gains, after-tax	0.3	(0.7)	0.0	(11.5)
Total Net Unrealized Gains (Losses) Reflected in Earnings, after-tax	(7.3)	(2.2)	162.1	(44.6)
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	13.9	1.4	35.9	(17.2)
Net Foreign Exchange Gains, pre-tax	0.0	0.0	0.0	(14.8)
Net unrealized investment gains (losses), pre-tax	13.9	1.4	35.9	(32.0)
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	0.2	0.7	0.2	0.0
Net Foreign Exchange Gains, pre-tax	0.0	0.0	0.0	0.0
Net unrealized investment gains (losses), pre-tax	0.2	0.7	0.2	0.0
Common equity securities
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(13.8)	5.5	71.7	(11.2)
Net Foreign Exchange Gains, pre-tax	0.0	0.0	0.0	0.0
Net unrealized investment gains (losses), pre-tax	(13.8)	5.5	71.7	(11.2)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(10.9)	2.3	101.8	0.4
Net Foreign Exchange Gains, pre-tax	0.3	(0.8)	0.0	3.3
Net unrealized investment gains (losses), pre-tax	$ (10.6)	$ 1.5	101.8	$ 3.7
MediaAlpha
Debt and Equity Securities, FV-NI [Line Items]
Unrealized investment gain			$ 114.7