Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Amortized Cost
Due in one year or less	$ 84.9
Due after one year through five years	457.3
Due after five years through ten years	1,500.7
Due after ten years	896.7
CMBS/RMBS/ABS/CDO/CLO	1,010.4	[1]
Total	3,950.0		3,402.6
Fair Value
Due in one year or less	86.9
Due after one year through five years	480.6
Due after five years through ten years	1,542.3
Due after ten years	930.2
CMBS/RMBS/ABS/CDO/CLO	1,036.2	[1]
Fair Value	$ 4,076.2		$ 3,426.3

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