Investing Activities Investments Classified by Contractual Maturity Date (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 36.1
Due after one year through five years	438.5
Due after five years through ten years	1,227.7
Due after ten years	778.0
CMBS/RMBS/ABS/CDO/CLO	922.3	[1]
Amortized Cost	3,402.6					2,798.1
Fair Value
Due in one year or less	36.8
Due after one year through five years	464.0
Due after five years through ten years	1,225.4
Due after ten years	769.8
CMBS/RMBS/ABS/CDO/CLO	930.3	[1]
Total	$ 3,426.3		$ 3,252.1	$ 3,162.2	$ 3,054.7	$ 2,972.3

[1] CMBS, RMBS, ABS, CDO and CLO are not listed separately in the table as each security does not have a single fixed maturity.