Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 50.8	$ (230.1)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(6.5)	34.4
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	44.3	(195.7)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.6)	2.8
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.6)	2.8
Net unrealized investment gains (losses), pre-tax	50.2	(227.3)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(6.5)	34.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	43.7	(192.9)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	17.5	21.7
Foreign Currency Transaction Gain (Loss), Unrealized	1.7	0.0
Net unrealized investment gains (losses), pre-tax	19.2	21.7
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	29.1	(249.8)
Foreign Currency Transaction Gain (Loss), Unrealized	0.5	2.4
Net unrealized investment gains (losses), pre-tax	29.6	(247.4)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	4.2	(2.0)
Foreign Currency Transaction Gain (Loss), Unrealized	0.2	0.4
Net unrealized investment gains (losses), pre-tax	4.4	(1.6)
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	(3.0)	0.0
Net unrealized investment gains (losses), pre-tax	$ (3.0)	$ 0.0