Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (230.1)	$ 15.2
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	34.4	(2.6)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(195.7)	12.6
Foreign Currency Transaction Gain (Loss), Unrealized	2.8	(5.0)
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	2.8	(5.0)
Net unrealized investment (losses) gains, pre-tax	(227.3)	10.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	34.4	(2.6)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(192.9)	7.6
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	21.7	11.4
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0
Net unrealized investment (losses) gains, pre-tax	21.7	11.4
Short-term investments
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	0.0	0.0
Net unrealized investment (losses) gains, pre-tax	0.0	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(249.8)	(2.8)
Foreign Currency Transaction Gain (Loss), Unrealized	2.4	(4.0)
Net unrealized investment (losses) gains, pre-tax	(247.4)	(6.8)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(2.0)	6.6
Foreign Currency Transaction Gain (Loss), Unrealized	0.4	(1.0)
Net unrealized investment (losses) gains, pre-tax	$ (1.6)	$ 5.6