Investment Securities (Net Realized and Unrealized 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]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 6.8	$ 0.0	$ 14.1
Net realized investment gains, pre-tax	2.7	42.3	259.5	57.1
Net unrealized investment gains (losses), pre-tax	25.4	(57.3)	(201.9)	(47.1)
Net realized and unrealized investment gains (losses), pre-tax	28.1	(15.0)	57.6	10.0
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(9.9)	4.5	(18.4)	(2.6)
Net realized and unrealized investment gains (losses), after tax	18.2	(10.5)	39.2	7.4
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	1.8	1.6	0.7	2.1
Net unrealized investment gains (losses), pre-tax	20.7	(13.4)	42.4	(2.0)
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	0.1	0.0	0.2	0.0
Net unrealized investment gains (losses), pre-tax	0.0	0.0	0.0	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	3.8	25.4	261.4	47.9
Net unrealized investment gains (losses), pre-tax	(10.6)	(21.0)	(258.0)	(27.7)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	(3.0)	15.3	(2.8)	7.1
Net unrealized investment gains (losses), pre-tax	$ 15.3	$ (22.9)	$ 13.7	$ (17.4)