Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (127.7)	$ 52.2	$ (118.2)	$ 81.2
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	36.0	(9.8)	33.2	(18.9)
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(91.7)	42.4	(85.0)	62.3
Foreign Currency Transaction Gain (Loss), Unrealized	(46.3)	31.2	(5.2)	44.3
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	11.5	(6.9)	1.2	(9.8)
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(34.8)	24.3	(4.0)	34.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(174.0)	83.4	(123.4)	125.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	47.5	(16.7)	34.4	(28.7)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(126.5)	66.7	(89.0)	96.8
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(34.7)	24.9	(22.6)	44.3
Foreign Currency Transaction Gain (Loss), Unrealized	(39.0)	28.2	1.2	40.4
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(73.7)	53.1	(21.4)	84.7
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.0	0.0	0.0	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(63.9)	21.0	(68.1)	25.0
Foreign Currency Transaction Gain (Loss), Unrealized	(8.2)	1.4	(7.5)	1.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(72.1)	22.4	(75.6)	26.9
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	(2.4)	0.7	(1.7)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.1)	0.3	0.0	0.3
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.1)	(2.1)	0.7	(1.4)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(29.1)	8.7	(28.2)	13.6
Foreign Currency Transaction Gain (Loss), Unrealized	1.0	1.3	1.1	1.7
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(28.1)	10.0	(27.1)	15.3
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	0.0	0.0	0.0	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	$ 0.0	$ 0.0	$ 0.0	$ 0.0