Investment Securities (Investment gains (losses) for Level 3) (Details) (Level 3 Inputs, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Fixed maturity investments.	Dec. 31, 2011 Fixed maturity investments.	Mar. 31, 2013 Common equity securities	Dec. 31, 2011 Common equity securities	Mar. 31, 2013 Other long-term investments	Dec. 31, 2011 Other long-term investments
Investment securities
Unrealized gains (losses) on investments	$ 6.5	$ 15.5	$ 0.2	$ 7.8	$ (0.1)	$ 0.5	$ 6.4	$ 7.2