Investing Activities - Maturities of Debt Securities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 46.2
Due after one year through five years	125.4
Due after five years through ten years	263.9
Due after ten years	1,664.4
CMBS/RMBS/ABS/CDO/CLO	782.5
Amortized Cost	2,882.4	2,798.1
Fair Value
Due in one year or less	47.1
Due after one year through five years	137.1
Due after five years through ten years	284.4
Due after ten years	1,769.7
CMBS/RMBS/ABS/CDO/CLO	816.4
Fair Value	$ 3,054.7	$ 2,972.3