Investing Activities Investments Classified by Contractual Maturity Date (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013
Amortized Cost
Due in one year or less	$ 67.3
Due after one year through five years	455.1
Due after five years through ten years	1,547.4
Due after ten years	1,029.7
CMBS/RMBS/ABS/CDO/CLO	961.4
Amortized Cost	4,060.9				3,390.6
Fair Value
Due in one year or less	68.4
Due after one year through five years	475.3
Due after five years through ten years	1,596.9
Due after ten years	1,083.6
CMBS/RMBS/ABS/CDO/CLO	997.6
Total	$ 4,221.8	$ 4,049.3	$ 3,829.6	$ 3,578.0	$ 3,416.1	$ 3,245.8	$ 3,154.8	$ 3,049.9