Fair Value (Carrying And Estimated Fair Values by Level Of Financial Instruments Not Recorded At Fair Value On Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	$ 2,442.1	$ 2,240.5
Short-term borrowings	1,180.0	440.0
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,442.1	2,240.5
Short-term borrowings	1,180.0	440.0
Commercial paper	1,315.5	629.0
Notes	13,399.0	14,323.8
Convertible debentures	336.2	334.1
Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,442.1	2,240.5
Short-term borrowings	1,180.0	440.0
Commercial paper	1,315.5	629.0
Notes	14,596.6	14,858.4
Convertible debentures	1,330.2	1,020.2
Estimated Fair Value [Member] | Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,442.1	2,240.5
Short-term borrowings	0.0	0.0
Commercial paper	0.0	0.0
Notes	0.0	0.0
Convertible debentures	0.0	0.0
Estimated Fair Value [Member] | Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	0.0	0.0
Short-term borrowings	1,180.0	440.0
Commercial paper	1,315.5	629.0
Notes	14,596.6	14,858.4
Convertible debentures	1,330.2	1,020.2
Estimated Fair Value [Member] | Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	0.0	0.0
Short-term borrowings	0.0	0.0
Commercial paper	0.0	0.0
Notes	0.0	0.0
Convertible debentures	$ 0.0	$ 0.0