Investment Securities (Net Realized and 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]
Trading Securities, Realized Gain (Loss)	$ 85.7	$ 27.2	$ 111.5	$ 52.5
Trading Securities, Realized Gain (Loss), Total	112.9	30.5	165.7	52.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(174.0)	83.4	(123.4)	125.5
Net Realized and Unrealized Gain (Loss) on Trading Securities	(61.1)	113.9	42.3	177.7
Income tax benefit (expense) attributable to net realized and unrealized investment gains (losses)	16.5	(23.0)	(9.8)	(38.7)
Realized and unrealized gain	(44.6)	90.9	32.5	139.0
Foreign Currency Transaction Gain (Loss), Realized	27.2	3.3	54.2	(0.3)
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(24.6)	(5.3)	(31.1)	(9.9)
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	(6.4)	(1.0)	(13.1)	(0.1)
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(31.0)	(6.3)	(44.2)	(10.0)
Trading Securities Realized Holding Gain (Loss) on Investments after Tax	61.1	21.9	80.4	42.6
Trading Securities, Realized Foreign Currency Gain (Loss) Net of Tax	20.8	2.3	41.1	(0.4)
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
Trading Securities, Realized Gain (Loss), Total	17.0	8.7	47.9	9.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(73.7)	53.1	(21.4)	84.7
Foreign Currency Transaction Gain (Loss), Realized	14.3	3.0	35.5	(0.5)
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
Trading Securities, Realized Gain (Loss), Total	5.7	0.0	9.3	0.0
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	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
Trading Securities, Realized Gain (Loss), Total	73.1	19.4	98.6	38.2
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(72.1)	22.4	(75.6)	26.9
Foreign Currency Transaction Gain (Loss), Realized	7.2	0.0	8.3	(0.1)
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
Trading Securities, Realized Gain (Loss), Total	0.6	1.5	(3.6)	3.9
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.1)	(2.1)	0.7	(1.4)
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
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
Trading Securities, Realized Gain (Loss), Total	16.9	0.9	13.9	0.6
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(28.1)	10.0	(27.1)	15.3
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.3	1.1	0.3
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)
Trading Securities, Realized Gain (Loss), Total	(0.4)	0.0	(0.4)	(0.1)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	0.0	0.0	0.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	$ 0.0	$ 0.0	$ 0.0	$ 0.0