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	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	$ 2,287.3	$ 2,240.5
Carrying Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,287.3	2,240.5
Short-term borrowings	540.0	440.0
Commercial paper	2,348.1	629.0
Notes	13,920.2	14,323.8
Convertible debentures	334.4	334.1
Estimated Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other invested assets, long-term	2,287.3	2,240.5
Short-term borrowings	540.0	440.0
Commercial paper	2,348.1	629.0
Notes	14,647.5	14,858.4
Convertible debentures	1,170.4	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	2,287.3	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	540.0	440.0
Commercial paper	2,348.1	629.0
Notes	14,647.5	14,858.4
Convertible debentures	1,170.4	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