Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 3.9	$ 0.0	$ 18.0
Net realized investment gains (losses), pre-tax	8.0	(7.4)	267.5	49.7
Net unrealized investment gains (losses), pre-tax	18.4	(36.5)	(183.5)	(83.6)
Net realized and unrealized investment gains (losses), pre-tax	26.4	(43.9)	84.0	(33.9)
Income tax (expense) benefit attributable to net realized and unrealized investment gains (losses)	(5.4)	9.4	(23.8)	6.9
Net realized and unrealized investment gains (losses), after tax	21.0	(34.5)	60.2	(27.0)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	2.8	(0.1)	3.5	2.0
Net unrealized investment gains (losses), pre-tax	(4.4)	3.3	38.0	1.3
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	0.2	0.0	0.4	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	6.4	(5.9)	267.7	42.0
Net unrealized investment gains (losses), pre-tax	12.7	(30.8)	(245.3)	(58.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains (losses), pre-tax	(1.4)	(1.4)	(4.1)	5.7
Net unrealized investment gains (losses), pre-tax	$ 10.1	$ (9.0)	$ 23.8	$ (26.4)