Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Amortized Cost
Due in one year or less	$ 107.6
Due after one year through five years	437.2
Due after five years through ten years	1,306.5
Due after ten years	736.5
CMBS/RMBS/ABS/CDO/CLO	915.4	[1]
Amortized Cost	3,503.2		3,402.6
Fair Value
Due in one year or less	111.1
Due after one year through five years	465.2
Due after five years through ten years	1,330.7
Due after ten years	747.8
CMBS/RMBS/ABS/CDO/CLO	934.3	[1]
Fair Value	$ 3,589.1		$ 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.