Investment Securities (Debt securities issued by corporation) (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]
Fair value investments	$ 5,957.2	$ 5,991.4	$ 6,736.7	$ 6,766.5
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,215.7	2,311.2
AAA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	194.1	236.9
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	818.2	957.8
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,188.8	1,105.9
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.2	0
Other | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 8.4	$ 10.6