Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 32.8
Due after one year through five years	153.5
Due after five years through ten years	298.4
Due after ten years	1,767.5
CMBS/RMBS/ABS/CDO/CLO	862.0
Total	3,114.2	2,798.1
Fair Value
Due in one year or less	33.5
Due after one year through five years	163.2
Due after five years through ten years	305.8
Due after ten years	1,784.7
CMBS/RMBS/ABS/CDO/CLO	875.0
Fair Value	$ 3,162.2	$ 2,972.3