Investment Securities (Investment gains (losses) for Level 3) (Details) (Level 3 Inputs, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (5.2)	$ 7.1
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(0.7)	0.2
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(1.8)	0.8
Convertible Debt Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	0.1	0
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (2.8)	$ 6.1