Investment Securities (Other Long-Term Investments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investment securities
Other Investments	$ 385.0	$ 262.3
Other long-term investments
Investment securities
Carrying value of investment accounted for using the equity method	23.2
Limited Liability Company and Private Equity Securities
Investment securities
Other Investments	69.7	20.3
Surplus Note
Investment securities
Other Investments	65.1	0
Total hedge and private equity funds included in other long-term investments
Investment securities
Other Investments	242.9	239.0
Other Debt Obligations [Member]
Investment securities
Other Investments	7.3	3.1
Forward contracts
Investment securities
Other Investments	0	(0.1)
Fair value measured on a recurring basis | Level 3 Inputs | Other long-term investments
Investment securities
Carrying value of investment accounted for using the equity method	$ 23.2	$ 26.6	$ 35.0