Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 26.4
Due after one year through five years	164.8
Due after five years through ten years	316.0
Due after ten years	1,858.7
CMBS/RMBS/ABS/CDO/CLO	841.3	[1]
Total	3,207.2		2,798.1
Fair Value
Due in one year or less	26.9
Due after one year through five years	175.2
Due after five years through ten years	325.8
Due after ten years	1,871.2
CMBS/RMBS/ABS/CDO/CLO	853.0	[1]
Fair Value	$ 3,252.1		$ 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.