Investments (Schedule Of Trading Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Equity securities and other invested assets [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	$ 1,157.9	$ 1,141.7
Trading securities, Amortized Cost	1,080.3	1,066.2
Equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	255.2	243.9
Trading securities, Amortized Cost	237.7	235.0
Other invested assets
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	902.7	897.8
Trading securities, Amortized Cost	842.6	831.2
Fixed Maturities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	6,473.1	6,737.7
Trading securities, Amortized Cost	6,535.6	6,847.4
U.S. government and government agencies
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,410.5	1,426.0
Trading securities, Amortized Cost	1,433.9	1,454.5
Non-U.S. government and government agencies
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	543.2	469.9
Trading securities, Amortized Cost	554.2	496.5
States, municipalities and political subdivisions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	41.7	354.1
Trading securities, Amortized Cost	40.7	355.8
Corporate Debt: Financial institutions
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,046.4	1,032.7
Trading securities, Amortized Cost	1,043.2	1,033.6
Corporate debt: Industrials
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	1,379.2	1,321.3
Trading securities, Amortized Cost	1,375.8	1,322.3
Corporate debt: Utilities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	153.2	140.0
Trading securities, Amortized Cost	153.3	140.7
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	595.6	614.5
Trading securities, Amortized Cost	593.9	612.5
Non-agency residential mortgage-backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	23.4	23.9
Trading securities, Amortized Cost	22.0	22.7
Commercial mortgage-backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	467.1	598.0
Trading securities, Amortized Cost	491.5	636.1
Asset-backed
Schedule of Trading Securities and Other Trading Assets [Line Items]
Trading securities, Fair Value	812.8	757.3
Trading securities, Amortized Cost	$ 827.3	$ 772.6