Investing Activities - Maturities of Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Amortized Cost
Due in one year or less	$ 33.5
Due after one year through five years	398.0
Due after five years through ten years	1,471.2
Due after ten years	1,558.2
CMBS/RMBS/ABS/CDO/CLO	786.7
Available-for-sale debt securities, amortized cost basis	4,247.6	$ 4,060.9
Fair Value
Due in one year or less	33.8
Due after one year through five years	414.6
Due after five years through ten years	1,500.5
Due after ten years	1,566.7
CMBS/RMBS/ABS/CDO/CLO	811.1
Fair Value	$ 4,326.7	$ 4,221.8