Investment Securities (Debt securities issued by corporation) (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 5,963.4	$ 5,991.4	$ 6,777.0	$ 6,766.5
Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	2,311.1	2,311.2
AAA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0.0	0.0
AA | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	200.5	236.9
A | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	887.6	957.8
BBB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,213.5	1,105.9
BB | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	6.4	0.0
Other | Debt securities issued by corporations
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 3.1	$ 10.6