Investments (Amortized Cost And Fair Value Of Fixed Maturity Securities, By Contractual Maturity) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	$ 732.1
Due after one year through five years, Amortized Cost	4,617.5
Due after five years through ten years, Amortized Cost	4,447.9
Due after ten years, Amortized Cost	3,226.0
Mortgage-backed securities, Amortized Cost	2,665.8
Available-for-sale Securities, Amortized Cost Basis, Total	16,587.1	16,440.5
Due in one year or less, Estimated Fair Value	754.1
Due after one year through five years, Estimated Fair Value	4,821.3
Due after five years through ten years, Estimated Fair Value	4,712.8
Due after ten years, Estimated Fair Value	3,375.1
Mortgage-backed securities, Estimated Fair Value	2,778.5
Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Amortized Cost Basis, Total	15,689.3	15,474.4
Total available-for-sale fixed maturity securities, Estimated Fair Value	$ 16,441.8