Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Amortized Cost
Due in one year or less	$ 97.7
Due after one year through five years	459.6
Due after five years through ten years	1,388.3
Due after ten years	756.8
CMBS/RMBS/ABS/CDO/CLO	990.2	[1]
Amortized Cost	3,692.6		3,402.6
Fair Value
Due in one year or less	100.5
Due after one year through five years	488.5
Due after five years through ten years	1,442.6
Due after ten years	787.4
CMBS/RMBS/ABS/CDO/CLO	1,021.9	[1]
Fair Value	$ 3,840.9		$ 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.