Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 18.1	$ (36.3)	$ (186.4)	$ (80.3)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(2.7)	7.9	22.2	22.6
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	15.4	(28.4)	(164.2)	(57.7)
Foreign Currency Transaction Gain (Loss), Unrealized	0.3	(0.2)	2.9	(3.3)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	0.3	(0.2)	2.9	(3.3)
Net unrealized investment gains (losses), pre-tax	18.4	(36.5)	(183.5)	(83.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(2.7)	7.9	22.2	22.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	15.7	(28.6)	(161.3)	(61.0)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(4.4)	3.3	38.0	1.3
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	0.0	0.0
Net unrealized investment gains (losses), pre-tax	(4.4)	3.3	38.0	1.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	12.5	(30.6)	(247.9)	(56.0)
Foreign Currency Transaction Gain (Loss), Unrealized	0.2	(0.2)	2.6	(2.5)
Net unrealized investment gains (losses), pre-tax	12.7	(30.8)	(245.3)	(58.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	10.0	(9.0)	23.5	(25.6)
Foreign Currency Transaction Gain (Loss), Unrealized	0.1	0.0	0.3	(0.8)
Net unrealized investment gains (losses), pre-tax	$ 10.1	$ (9.0)	$ 23.8	$ (26.4)