Investments (Amortized Cost And Fair Value Of Fixed Maturity Securities, By Contractual Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Available-for-sale securities, Amortized Cost	$ 19,668.4	$ 18,619.7
Available-for-sale securities, Estimated Fair Value	20,355.1	19,187.4
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Due in one year or less, Amortized Cost	373.1
Due after one year through five years, Amortized Cost	4,411.8
Due after five years through ten years, Amortized Cost	5,270.0
Due after ten years, Amortized Cost	6,136.8
Mortgage-backed securities, Amortized Cost	2,059.1
Available-for-sale securities, Amortized Cost	18,250.8	17,516.4
Due in one year or less, Estimated Fair Value	374.3
Due after one year through five years, Estimated Fair Value	4,489.4
Due after five years through ten years, Estimated Fair Value	5,379.6
Due after ten years, Estimated Fair Value	6,197.8
Mortgage-backed securities, Estimated Fair Value	2,068.6
Available-for-sale securities, Estimated Fair Value	$ 18,509.7	$ 17,687.5