Investment Securities (Net Realized 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]
Trading Securities, Realized Gain (Loss)	$ 256.8	$ 14.3
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(42.9)	(4.5)
Trading securities realized holding gain (loss) on investments after tax	213.9	9.8
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.5
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	0.0	0.5
Net realized investment gains, pre-tax	256.8	14.8
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(42.9)	(4.5)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	213.9	10.3
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(1.1)	0.5
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0
Net realized investment gains, pre-tax	(1.1)	0.5
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.0	0.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0
Net realized investment gains, pre-tax	0.0	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	257.6	22.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.5
Net realized investment gains, pre-tax	257.6	22.5
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.3	(8.2)
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0
Net realized investment gains, pre-tax	$ 0.3	$ (8.2)