Investment Securities (Corporate debt securites) (Details) (Debt securities issued by corporations, USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 755.7	[1]	$ 754.5	[1]
AA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	47.1		51.2
A
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	352.2		357.9
BBB
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	355.6		345.4
BB
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 0.8		$ 0

[1] Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor’s Financial Services LLC (“Standard and Poor’s”) and 2) Moody’s Investor Service (“Moody’s”).