Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (37.9)	$ 36.2
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	6.3	(3.7)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(31.6)	32.5
Foreign Currency Transaction Gain (Loss), Unrealized	(10.7)	(0.5)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(10.7)	(0.5)
Net unrealized investment (losses) gains, pre-tax	(48.6)	35.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	6.3	(3.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(42.3)	32.0
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(18.6)	10.2
Foreign Currency Transaction Gain (Loss), Unrealized	(14.8)	1.7
Net unrealized investment (losses) gains, pre-tax	(33.4)	11.9
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 (losses) gains, pre-tax	(0.7)	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(16.7)	19.1
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.5
Net unrealized investment (losses) gains, pre-tax	(16.7)	19.6
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(1.9)	6.9
Foreign Currency Transaction Gain (Loss), Unrealized	4.1	(2.7)
Net unrealized investment (losses) gains, pre-tax	$ 2.2	$ 4.2