Investments (Amortized Cost And Fair Value Of Fixed Maturity Securities, By Contractual Maturity) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Total available-for-sale securities, Amortized Cost	$ 18,402.1	$ 17,356.1
Total available-for-sale securities, Estimated Fair Value	19,673.3	18,586.9
Fixed Maturity Securities [Member]
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	812.5
Due after one year through five years, Amortized Cost	4,914.9
Due after five years through ten years, Amortized Cost	4,891.3
Due after ten years, Amortized Cost	4,048.0
Mortgage-backed securities, Amortized Cost	2,804.4
Total available-for-sale securities, Amortized Cost	17,471.1	16,459.1
Due in one year or less, Estimated Fair Value	826.5
Due after one year through five years, Estimated Fair Value	5,136.5
Due after five years through ten years, Estimated Fair Value	5,188.9
Due after ten years, Estimated Fair Value	4,226.0
Mortgage-backed securities, Estimated Fair Value	2,909.8
Total available-for-sale securities, Estimated Fair Value	$ 18,287.7	$ 17,344.4