Investment Securities (Other long-term investments) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 402.6	$ 407.0
Partnership investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	3.8	5.2
Hedge and private equity funds included in other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	242.5	242.9
Surplus Note
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	68.0	65.1
Limited Liability Companies and Private Equity Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	66.6	69.7
Affordable Housing Development Fund [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	16.2	16.8	18.5	19.1
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Investments	$ 5.5	$ 7.3