Investment Securities (Net Realized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	$ 2.7	$ 42.4	$ 259.5	$ 56.7
Trading securities, Tax on Realized Holding Gain (Loss) on Investments	(0.4)	(12.7)	(43.3)	(17.2)
Trading securities realized holding gain (loss) on investments after tax	2.3	29.7	216.2	39.5
Foreign Currency Transaction Gain (Loss), Realized	0.0	(0.1)	0.0	0.4
Trading Securities, Tax on Realized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0	0.0
Trading securities, realized foreign currency gain (loss) net of tax	0.0	(0.1)	0.0	0.4
Net realized investment gains, pre-tax	2.7	42.3	259.5	57.1
Tax on Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities	(0.4)	(12.7)	(43.3)	(17.2)
Net Realized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	2.3	29.6	216.2	39.9
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	1.8	1.6	0.7	2.1
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Net realized investment gains, pre-tax	1.8	1.6	0.7	2.1
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	0.1	0.0	0.2	0.0
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Net realized investment gains, pre-tax	0.1	0.0	0.2	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	3.8	25.5	261.4	47.5
Foreign Currency Transaction Gain (Loss), Realized	0.0	(0.1)	0.0	0.4
Net realized investment gains, pre-tax	3.8	25.4	261.4	47.9
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading Securities, Realized Gain (Loss)	(3.0)	15.3	(2.8)	7.1
Foreign Currency Transaction Gain (Loss), Realized	0.0	0.0	0.0	0.0
Net realized investment gains, pre-tax	$ (3.0)	$ 15.3	$ (2.8)	$ 7.1