Investments (Schedule Of Trading Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Equity securities and other invested assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	$ 1,043.2	$ 1,243.2
Trading securities, Amortized Cost	978.7	1,166.2
Equity securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	216.1	403.0
Trading securities, Amortized Cost	208.0	395.3
Other invested assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	827.1	840.2
Trading securities, Amortized Cost	770.7	770.9
Fixed maturity investments [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	7,327.3	7,201.5
Trading securities, Amortized Cost	7,311.5	7,290.6
U.S. government and government agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,471.2	1,434.0
Trading securities, Amortized Cost	1,466.1	1,438.0
Non-U.S. government and government agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	488.1	556.8
Trading securities, Amortized Cost	498.2	579.2
States, municipalities and political subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	480.8	413.5
Trading securities, Amortized Cost	460.3	396.0
Corporate debt, Financial institutions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,084.2	1,275.4
Trading securities, Amortized Cost	1,066.8	1,277.3
Corporate debt, Industrials [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,372.5	1,308.1
Trading securities, Amortized Cost	1,353.5	1,345.6
Corporate debt, Utilities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	138.4	118.9
Trading securities, Amortized Cost	136.2	125.4
Agency mortgage-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	845.9	751.8
Trading securities, Amortized Cost	831.0	745.5
Non-agency residential mortgage-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	22.6	34.0
Trading securities, Amortized Cost	21.5	32.4
Commercial mortgage-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	631.8	582.8
Trading securities, Amortized Cost	671.3	600.1
Asset-backed [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	791.8	726.2
Trading securities, Amortized Cost	$ 806.6	$ 751.1