Investment Securities (Debt securities issued by corporation) (Details) (Debt securities issued by corporations, USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,431.4	$ 2,347.2
AAA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
AA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	220.6	228.8
A
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,037.6	1,039.5
BBB
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	1,167.1	1,075.5
BB
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	0	0
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 6.1	$ 3.4