Investment Securities (Net Realized 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]
Trading Securities, Realized Gain (Loss)	$ 85.7	$ 27.2	$ 111.5	$ 52.5
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(24.6)	(5.3)	(31.1)	(9.9)
Trading securities realized holding gain (loss) on investments after tax	61.1	21.9	80.4	42.6
Foreign Currency Transaction Gain (Loss), Realized	27.2	3.3	54.2	(0.3)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	(6.4)	(1.0)	(13.1)	(0.1)
Trading securities, realized foreign currency gain (loss) net of tax	20.8	2.3	41.1	(0.4)
Trading Securities, Realized Gain (Loss), Total	112.9	30.5	165.7	52.2
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(31.0)	(6.3)	(44.2)	(10.0)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	81.9	24.2	121.5	42.2
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	2.7	5.7	12.4	10.1
Foreign Currency Transaction Gain (Loss), Realized	14.3	3.0	35.5	(0.5)
Trading Securities, Realized Gain (Loss), Total	17.0	8.7	47.9	9.6
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	5.7	0.0	9.3	0.0
Trading Securities, Realized Gain (Loss), Total	5.7	0.0	9.3	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	65.9	19.4	90.3	38.3
Foreign Currency Transaction Gain (Loss), Realized	7.2	0.0	8.3	(0.1)
Trading Securities, Realized Gain (Loss), Total	73.1	19.4	98.6	38.2
Convertible fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.6	1.5	(3.6)	3.9
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Trading Securities, Realized Gain (Loss), Total	0.6	1.5	(3.6)	3.9
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	16.9	0.6	12.8	0.3
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.3	1.1	0.3
Trading Securities, Realized Gain (Loss), Total	16.9	0.9	13.9	0.6
Forward Contracts
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(0.4)	0.0	(0.4)	(0.1)
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Trading Securities, Realized Gain (Loss), Total	$ (0.4)	$ 0.0	$ (0.4)	$ (0.1)