Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 9.9	$ 19.8	$ (28.0)	$ 56.1
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(11.4)	1.0	(5.1)	(2.6)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(1.5)	20.8	(33.1)	53.5
Foreign Currency Transaction Gain (Loss), Unrealized	(0.8)	0.5	(11.5)	(0.1)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.1	0.0	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.7)	0.5	(11.5)	(0.1)
Net unrealized investment gains (losses), pre-tax	9.1	20.3	(39.5)	56.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(11.3)	1.0	(5.1)	(2.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(2.2)	21.3	(44.6)	53.4
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	1.4	7.3	(17.2)	17.5
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	5.8	(14.8)	7.4
Net unrealized investment gains (losses), pre-tax	1.4	13.1	(32.0)	24.9
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.7	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0
Net unrealized investment gains (losses), pre-tax	0.7	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	5.5	7.8	(11.2)	26.9
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	2.6	0.0	3.1
Net unrealized investment gains (losses), pre-tax	5.5	10.4	(11.2)	30.0
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	2.3	4.7	0.4	11.7
Foreign Currency Transaction Gain (Loss), Unrealized	(0.8)	(7.9)	3.3	(10.6)
Net unrealized investment gains (losses), pre-tax	$ 1.5	$ (3.2)	$ 3.7	$ 1.1