Investments (Maturities Of Available-For-Sale Debt Securities) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Investments [Abstract]
Due in one year or less, Amortized Cost	$ 441.4
Due in one year or less, Fair Value	446.2
Due after one year through five years, Amortized Cost	2,092.0
Due after one year through five years, Fair Value	2,188.4
Due after five years through ten years, Amortized Cost	1,592.2
Due after five years through ten years, Fair Value	1,720.2
Due after ten years, Amortized Cost	651.3
Due after ten years, Fair Value	700.6
Total fixed maturities, Amortized Cost	4,776.9
Total fixed maturities, Fair Value	5,055.4
Mortgage-backed and asset-backed securities, Amortized Cost	1,364.3
Mortgage-backed and asset-backed securities, Fair Value	1,415.2
Total fixed maturities, Amortized Cost	6,141.2	6,008.7
Fixed maturities, Fair Value	$ 6,470.6	$ 6,284.7