Investment Securities (Investment Gains (Losses) For Level 3) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011	Dec. 31, 2012
Investment securities
Realized gains	$ 270.0	$ 221.4	$ 162.2
Realized losses	36.2	116.9	94.1
Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	3.7	9.8		17.7
Fixed maturity investments | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	1.2	(2.3)		7.7
Common equity securities | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	5.9	0.9		3.0
Convertible fixed maturity and preferred investments | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	0.7	0		0
Other long-term investments | Level 3 Inputs
Investment securities
Total pre-tax gains (losses) included in earnings attributable to unrealized investment gains (losses) for Level 3 investments	$ (4.1)	$ 11.2		$ 7.0