Investment Securities (Investment gains (losses) for Level 3) (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
Unrealized gains (losses) on investments	18.1	(36.3)	(186.4)	(80.3)
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(4.4)	3.3	38.0	1.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	12.5	(30.6)	(247.9)	(56.0)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	10.0	(9.0)	23.5	(25.6)
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	8.1	(6.7)	22.3	(3.5)
Level 3 Inputs | Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.3	(1.0)	1.6	(0.9)
Level 3 Inputs | Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.0	1.1	0.0	4.0
Level 3 Inputs | Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 7.8	$ (6.8)	$ 20.7	$ (6.6)