Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Amortized Cost
Due in one year or less	$ 51.3
Due after one year through five years	427.9
Due after five years through ten years	1,518.8
Due after ten years	1,319.7
CMBS/RMBS/ABS/CDO/CLO	921.1
Total	4,238.8	4,060.9
Fair Value
Due in one year or less	51.9
Due after one year through five years	449.1
Due after five years through ten years	1,592.6
Due after ten years	1,401.5
CMBS/RMBS/ABS/CDO/CLO	963.1
Fair Value	$ 4,458.2	$ 4,221.8